The old Football Combination League was a really strong reserve league until the advent of the Premier League. Worked both ways, likes of Arsenal, Spurs and West Ham put out their young players or experienced out of favour 1st teamers who would play against lower league players who had played 1st team football with smaller squads then and would be competitive and maybe a bit robust.
The old Football Combination was brilliant, I remeber when Spurs brought a near first team to the CG back in the very early 80's it had Ricky Villa, Glenn Hoddle, Garry Brook, Barry Daines and Micky Hazard all playing who were all regulars for Spurs at the time after fitness, we drew 2-2 in front of a crowd of about 5,000 with just the North Stand open.
Arsenal, Spurs, Chelsea and West Ham used to reguarly play first team players back then and was a good way for youngsters to get a feel of playing against better quality players before hitting the first team.