The casebacks are stainless steel in either instance, you wouldn't want the plate in contact with skin for wear, plus there may be some nickel in the chrome plate and that's an irritant. And of course it saves Heuer money to share casebacks through the different models. Same thinking behind s/s casebacks on gold-plated models.

It's quite possible Hamilton only took the steel cases, but I gave up attributing rhyme and reason to this sort of thing a while ago!