The 235 rings did change a couple of times. For '41-'49, both compression rings were taper face 1/8". For '50-'53, the upper compression ring changed to a twist-type 3/32". For '54-'63, both compression rings were twist-type 3/32". All oil rings were 3/16".
The 235 pistons and rods changed too. '53-down, the rod was 6 13/16", the piston compression distance was about 2 1/16", the piston length was 3.75", and the piston pin diameter was around .8645". '54-up, the rod was 6.81", the piston compression distance was 2.08", the piston length was 3.58", and the piston pin was slightly bigger at .866". Of course, we all know the rod bearings changed to inserts for '54-up. Also, the pistons changed from cast iron to cast aluminum for '54-up
Again, I don't think any of this really matters, as long as the replacement rings go with whatever replacement piston is obtained. If you only want replacement rings to use on the existing pistons, that's another story. The only thing I'm not quite sure of is if the piston end of the earlier connecting rod would expand enough to fit the larger pin of the later piston. It's only a .001"-.0015" difference, so it probably wouldn't be a problem. I suppose the compression distance should be checked, too, to be sure the piston doesn't stick above the block deck, though were only talkin' about .0175" of difference. One advantage of the larger compression distance on the later piston would be that the compression ratio might be raised ever so slightly.
All this info is believed to be correct, and was sourced from the '48-'51 shop manual, the '53 and '54 truck resto packs, and a '64 Chevy master parts catalog.
Sealed Power makes a '53-'62 235 piston, cast aluminum, flat top, 3/32, 3/32, 3/16 rings, and 2.062" (same as the early piston) compression distance. It's part #1009P with pins (they don't mention pin diameter), available in std and 5 oversizes. Only $98 a set at Fast Engine Parts. They have matching rings too, of course.
