It's fairly new, came out after the catalog was printed.
My buddy of 30 years bought one for CC, I shot it at the range, and really liked it. I have a Smith M-16 in .32H&R and a Ruger Single Six Bisley as well, I like the .32. This new .327 has plenty of zip, but easier to handle in a small frame than .357
Can't say much about Ruger, but I chose a S&W 638 to have the option of SA and can't praise it enough as a pocket gun. Only con I've run into is buying a crimson lg405 that I thought was a full wrap-around stock as they sort of describe - it's not and has lousy plastic side panels - when I emailed crimson they said so sorry and that was it. It really screwed up the feel of the grip, but guess I can live with that since it's easier to put the dot on them than try for a sight picture in fast action.
My next look will be at the S&W 640 or 649 for a .357, most probably without the crimson grip laser.
Yeah .... my S&W 438 is the same as your 638 - except for color -- 438 = black and I think 638 = stainless. Having the shrouded hammer is interesting. Allows a hammer cock with a very light trigger release. Makes for some careful shots. I like it.