New Style Hand vs. Old Style Hand for 686-6

[My Toy]

I have a 686-6 that the cylinder does not carry up far enough to engage the bolt before the hammer drops. Have had this problem on some of the 686's I've collected over the years (embarrassed to say how many I own).
Anyway there are 2 styles of hand for the 686. See attached pics. The older style hand part# 7522 which seems to have been used on revolvers up to revision 3 and the newer style hand part# 21062 which appears to be used after revision 3. I have tried both styles in the 686-6 and both seem to function OK. My question to Smith and Wesson revolver aficionados is -- are either style hand techniquely interchangeable in the newer revision revolvers and what is or was the reason for the design change. It appears to me the newer style hand has a hump on the front edge whereas the older style hand is a little wider (front to back) in the midsection. See attached pics.

 

[MygunisaS&Wrevolver]

That's a great question that I have also wondered about. A guess on my part is they have incorporated an additional let out surface so the humped hand might also be used in the newer L frame seven shooters. Hopefully one of the gunsmithing experts will reply with a definitive answer?

 

[My Toy]

I called S&W customer service with the question a couple of weeks ago. The customer service person put me on hold for quite a while and when she came back with the answer it was really to something I didn't ask. So I didn't think it was worth a retry since it seemed like I had used up my allotted time for question.

I'm still hoping there is someone in S&W land that has in the past researched the reason for the difference in design between these two hands.