They weren't even allowed to play the kids:-
Premier League rules
E. 20
In every League match each participating club shall field a full strength team.
B. 13
In all matters and transactions relating to the League each club shall behave towards each other club and the League with the utmost good faith.
Revie getting £5,000 fine for fielding a weakened team against Derby County in 1970. And Wolves and Blackpool fined £25,000 for the same offence as late as 2009 and 2010.
The rule change in 2011 to allow any selection from the named squad is still ambiguous though. There is some allowance for playing under 21's not in the squad but the rule still applies if the number of under 21's fielded were considered excessive.
Remember Leeds getting fined for that. If I call correctly all of the weakened team had at least one international cap. Some clown said regarding the fixture build up which caused it "If these teams will get involved in European competitions that is their lookout...".
How times change, unlimited replays for FA Cup, the League Cup and a 42 game league season plus European games for the top clubs. 60+ games were routine and even 70+ on occasion. Admittedly there weren't as any European Cup Games as now but a larger number of teams were involved in Europe making more teams subject to fixture congestion atbonevteam Leeds and others were playing Saturday, Tuesday (often in Europe), Thursday and Saturday. Leeds had to play the game which decided whether they won the League or not the Monday after the played in the Cup Final Don't like Leeds but that was ludicrous. Yet in those tomes in the late 60s and 70s these clubs were successful in Europe, often playing more games than nowadays whilst using fewer players.