There's no limit to how far back this can go. It's down to a club to make a submission to the EFL and pay for the medals once they've been approved by the league statisticians. The rags and both Merseyside clubs have all got involved in this in relation to old title wins.
I'd imagine there are fewer players eligible than people might imagine as squad rotation didn't happen back then and there were no subs until the mid 60s, only one per game for a long time after that and for quite a few years he'd only get on the pitch if there was an injury. They did play 42 games per season though rather than 38.