Subjective opinion of top highest average level clay court players Pre Open Era in chronological order
HL Doherty
Wilding
Bill Tilden
Rosewall
Laver
Gonzalez could be there too as well as well as Tony Trabert. I understand Segura was pretty good on clay also but I have to check his record on that surface..
I think that you mean the 1950's, when the clay field was the toughest ever, much tougher than today.
Just think:
Segura, Patty, Drobny, Trabert, Rosewall, Hoad, Davidson, Pietrangeli to name only the greatest. Nadal would have won only one French title at most in the 1950's. If VERY lucky, two.
No sarcasm.