Meanwhile I contacted LFL about the 26-10-1960 Netherlands B - Luxembourg match they labelled as South Netherlands - Luxembourg and I am waiting for their reply.
I have analyzed several B-teams line ups and here a summary of what I have found.
Austria: Most of the B-teams that played against other countries A-teams contained at least six players who have played already A-international matches. Even though played on the same day Austrian A-national team came in action the teams fielded experienced players. The team that played Luxembourg on 20-03-1949 fielded seven players who had played already A-international matches before.
Most experienced team however, was the Austrian B-side that played against Netherlands on 22-05-1952. Six of the players already appeared in A-international matches before. Goalkeeper Zeman already played 31 full internationals. Kowanz (16) and Wagner (19) were the other experienced players. Even though Austria did not played on the same day the selection was a complete different one as Austrian A- team that played England three days later.
Belgium: In the 1928-34 era mostly very unexperienced teams with only three players who have played full A-internationals before. But always fielding also players who would win A-caps later, mostly within a year and sometimes also in the next A-international match Belgium will play. Notably the most experienced of these sites, that played on 04-11-1928 against Luxembourg obtained the worst result: 1-1.
During the preparation for 1934 World Cup a Belgian B-side fielded nine players who have already won full caps when they beat Luxembourg 4-1 on 10-05-1934. Uncapped player De Vries, however, will make his full international debut seventeen days later in Belgium’s next match. From that moment on until WW 2 Belgium B-teams will field most of the times at least six players who have already played official A-international matches.
In the first years after WW 2 Belgian B-teams are less experienced, but most of the times at least four players had already played A-internationals. It is remarkable in the 1945-48 era notably the two most experienced teams are not able to beat Luxembourg. On 04-05-1947 a Belgium B-side fielding five A-international players drew 3-3 with Luxembourg. When Belgium B fielded six A-international players on 22-02-1948 they even lost 2-0 against Luxembourg.
Czechoslovakia: Most of its B-teams fielded at least five players who have already played A-inernational matches. The team that beat Indonesia 5-1 on 26-09-1956 fielded even three players who already have won at least ten caps.
England: The England B-team that played against another coutries A-teams mostly fielded at least five players who have already played full A-internationals. Sometimes there are very unexperienced sides as against Netherlands (26-03-1952), New Zealand (15-10-1979) Malta (14-10-1987) with only 2, 1 and even no players who have already played full internationals. The team that played against Algeria (11-12-1990) and Iceland (27-04-1991) fielded 6 and 7 players who have played A-internationals before. Also the team that made the tour to Down Under and Asia in 1978 was relatively experienced, always fielding at least five players who played full internationals before.
In 1950 England used its B-team to prepare for the 1950 World Cup tournament. The team that played against Netherlands on 22-02-1950 fielded four A-international players, the team that toured (simulatneously with England A) the Continent three months later fielded mostly five A-international players and two players who made theire full internaional debut at the World Cup tournament. Though England B was disappointing (5-0 defeat against Italy B (de facto Italy weakened A-team), 3-0 defeat against Netherlands, 2-1 win against Luxembourg (after being 1-0 down at half time)) seven players who toured with the England B-team were selected for the 1950 World Cup team.
France: far most of the matches with players who never won a full cap before, but also the B-team with most players that will make their full international debut soon. Sometimes France B fielded a team without any player that apperead in an A-international match before, which is exceptional if we compare the French B-teams with other countries B-teams.
Italy: Italian B-teams are one of the least experienced B-teams except the team that played Luxembourg on 15-05-1938 which fielded four players that already played A-international matches. Eight of that team were drafted into the squad that will win the World Cup five weeks later. Colaussi, who scored twice in the final against Hungary, is one of them. Usually not more than two – very unexperienced – players appeared in Italian B-team. As in other countries B-teams most of the uncapped players will play official A-internationals later, only a few uncapped Italy B-players made their A-international debut later.
Italy's B-team that beat England B on 11-05-1950 was rather a weakened A-team than a B-team. It was a preparation match for the 1950 World Cup, ten of the Italian players were selected for the Italian World Cup squad, seven of them haf played A-internationals previously. Engeland B was also a mix of A- and B-players.
Netherlands: Most of the Dutch B-teams fielded four players who have already played A-inernationals including also some experienced A-international players. Sometimes the B-team is more than just a reserve team.
The team that beat USA 3-2 on 01-06-1928 consisted of players who were part of the Dutch 1928 Olympic Games selection as yoy can read here http://roonba.20.forumer.com/viewtopic.php?t=1068
The Dutch team is labelled as B-team which you can read here too. http://nl.wikipedia.org/wiki/Lijst_van_voetbalinterlands_Nederlands_B-voetbalelftal The book Oranje Toen en Nu deel 3 1927-1932 and 2002/2003 reports on page 59:
Amsterdam, 01-06-1928: Netherlands 3-2 USA; Dutch goalscorers: Freese (2), Van der Griend; spectators: 100 (one hundred)
Netherlands: De Boer (Ajax): Van Run (PSV), Van der Zalm (VUC); Kools (NAC), Vis (HVV), Krom (RCH); Smeets (HBS), Tap (ADO), Freese (Heracles), Van der Griend (Hermes DVS), Weber (Sparta).
The book mentioned the reserves team that beat USA 3-2 was reinforced with some A-players. All players that played against USA were part of the Dutch selection for the 1928 Olympics. Freese and Ghering played on 30-05-1928 against Uruguay, the other nine were the non playing reserves. After the elimination the Dutch played against Belgium (05-06-1928 and Chile (08-06-1928 for the consolation tournament and an extra friendly against Egypt (14-06-1928. In these matches Kools, Krom, Smeets, Tap and Weber also played. Van der Griend also played two full A-internationals against Belgium (11-03-1928 and 01-04-1928.
Actually the Dutch intended to play with many reserves the 1928 Olymic Games consolation tournament. But plans changed when the Dutch had to play Belgium. As this used to be a very prestigeous match the Dutch decided to field a full strengthened team: Kools, Krom, Smeets, Tap and Weber (who played against USA) were also included in the team that played against Belgium.
The team that beat Luxembourg 1-0 on 27-03-1948 fielded only three players that played A-international matches before, but three of them made their official A-international debut in the first A-international Netherlands would play (on 18-04-1948 against Belgium). Eight of the players who appearde against Luxembourg played official A-international matches in the 14-03-1948 – 09-06-1948 era.
The team that beat Luxembourg 3-0 on 29-09-1965 was a mix between Ateam. Dutch team manager Dennis Neville used this match to test some players for the crucial World Cup qualifying matches against Switzerland in October and November. Actually five players who had already played A-international matches would have been fielded, but eventually four of them played.