What does the - number mean re smith and wesson 629 specs

[gaucho]

I have began looking for a good used Smith 44 Mag 4 inch bbl. I notice in the ads a "dash" designator, for instance 629-1, -6 etc. I assume these refer to model upgrades. If so, I have heard some negative info about some MIM parts in some newer models, so the question is, what model designation are before S&W added the MIM parts that I want to avoid?

Thanks in advance for your help.