Pure chance. Depends on the individual gun. My 638 purchased in 2011 (IIRC) has been reliable and was my casino go-to gun before the 340s arrived. OTOH, I inspected a 638 several years later and the hammer could not be fully pulled back by hand, perhaps a burr on the interior. Maybe that 'burr' would wear off under firing, but perhaps not. You can't have maybes on a carry piece.
Don't know if this means anything, but S&W produces a huge number of their Centennial pattern guns (442, 642, 340), to the point that I rarely see a 638 in a LGS. All you can do is carefully inspect, but the acid test is at the range.
Kaaskop49
Shield #5103