As noted, no internal lock on a 19-6; MIM came in late in the -7s and the -8s were discontinued in '99 (with a brief special run in 2000), so I don't believe any 19 had a lock, unless you count the also brief run of the .38 Special only 19 "Classic" of a couple years ago.
I have a pet theory on S&Ws from the late '80s early '90s period in which the 19-6 in question was made: they're some of the best S&Ws ever. It was a transition period at S&W and saw an overlap between the old school artisan handfitters side-by-side with modern CNC machining -- best of both worlds. And they're overlooked by many collectors and accumulators because they're not from the pinned-and-recessed era, so availability is greater and cost is lower.
One mechanical issue to be aware of on -6s: S&W's brief flirtation with a floating hand (rather than a fixed). In short, the floating hand was an attempt at a self-adjusting hand that could "move" with the onset of wear on the extractor notches or frame window; the design was problematic enough that it was discontinued after a couple years; many floating hands had to be converted to fixed hands (an easy and inexpensive switch). A few work fine and are problem free still.
If the 19-6 you shot has no trigger issues, it may be converted to fixed, or it may be one of the floating hands that work. Either way, it shouldn't discourage you from obtaining this particular 19-6 if you like it -- just worth knowing about.
Good luck and enjoy.