There’s definitely a difference for sure. See I’ve noticed that the metal is a lot easier to bend, whenever I get a pre-19 cap I always notice the extra force needed to adjust the fit. They seem tougher too - drops that would dent a new cap often don’t dent an old cap. Not that they’re dent proof of course.I'd love to get a true analysis of the material to know what the actual difference is. I suspect it has to do with chromium content in the stainless steel. I also suspect that they are harder to form during fabrication. They probably came from an alternate supplier and that is why there is so much variation.
The thickness seems similar, which leaves the composition of the alloy as the potential culprit like you say. They seem to perform similar with a torch, but IH is where you really notice the new cap/old cap gap