I have been filing contest reports from the 1960s, and indeed a 200 pound curl IN COMPETITION is rare. It appears most curl competitions were done standing (of course) without the back braced against some upright item.
Luther Rogers curled 235 on Nov 19, 1960 and again on Jan 1, 1964 in competition.
Several other men also managed 200 in competition:
Winston Binney
Pat Casey
Val Vasilieff (later changed the spelling of his name)
George Gillow
Mike Giardina
Also rare is a bodyweight curl, but some of the lighter men could curl bodyweight, or slightly more.
These lifts were for the first half of the 1960s- more may be encountered as I complete the decade.