Per pages 258 & 259 of the Standard Catalogue of S&W, 3rd edition:
Model 629-3 Carry Comp II - slightly different front sight (which yours seems to be, to me), compensator, 3" barrel, unfluted cylinder - 100 manufactured for Lew Horton
I'm not saying the gun in question is definitely one of these, but it seems to fit the bill. I think the Lew Horton website has a list of serial numbers for their models, you could check there. Or call S&W.