No, I didn't forget that at all. Just because they only had one sub (or no sub at all) they could still have rotated their squads, but they didn't. There was the first XI and the reserves, and most of the time that's the way it stayed.
Up until the 60s or 70s that was pretty much the norm. Every club had a first team, and the reserve RB, LB, left half, inside right or whatever came in if the first teamer was injured. Occasionally a reserve would force their way into the first team if the player in possession went off his form...