From what little I know, about the guns and the fact I own a 625 three inch barrel (just got it), 4 inch barrel and also a mountain gun, I would say, get a pre lock of you preference of barrel, length. Two are -3's and one is -6 (mountain gun)
Functionally between the -2, -3, -4, -5, -6 and what ever, for use I do know that much difference other than firing pin and when MIM was started to be used. Again, I am not qualified for technical differences. I just prefer the prelock's maybe they look or feel to me to be a bit better built.
I am not a purist, but functionally yet to have an issue. Between the two 4 inch guns after close to 1000 out of each. I have not fired the 3 inch enough yet but I really,

like it.
The biggest issue is this unique items reproduce at the most in opportune times.

They love to feed on plastic cards that hide in wallets, sheet of paper in checkbooks, and just madly consume the government debit cards (formally called cash before the stimulus plan)
Best in your choice.
Be safe and keep the brass flying
Terry Peters
Do your research but you get what you pay for front end or back end.