As I recall the SuperTiger C35 and Stunt 35 (plain bearing) came on the market in the early 1960's.
They were soon followed by the ball bearing G21.
Some time before that there was an older G21 with a front & back plate. I never saw one of these is use, but only displayed by collectors.
Over the years, G21's were produced in short production runs with changes every time. There didn't seem to be any change in the names, always G21./35. I remember PDP (Perry directional porting) as being one of the later revs. Many of the revs involved muffler ears and other changes in favor of RC, which had become the focus of the ST business. I think PDP was a low cost way of competing with schunural porting without making a whole new engine.