That's a screaming deal. The rounded trigger guard 639 is an earlier pistol, I would guess an A-prefix gun rather than a three alpha prefix, which puts production far closer to 1983 or '84. By 1989 the Model 639 was phased out in favor of the 3rd Gen pistols. Even still, I have seen the MSRP for a fixed-sight Model 639 listed as $523.50. If you put that 1989 money in to an inflation calculator... your $379 price should make you stand up and DANCE!
Your new gun shares stocks/grips with the 39/39-2, 439 and the elite Model 52 pistols. The 639 has an internal firing pin lock that the 39/39-2 did not have. It's an all stainless pistol and it's heavy and solid.
Some 639's came with ambidextrous decocker levers, some earlier pistols did not. Original magazines for the 639 are silver in color but seem otherwise basically the same as the later model 39-2 magazines... they hold 8 rounds, have a plastic follower and a steel floor plate. The newer 3rd Gen magazine is quite still often found (even new in package!) and is typically marked as a "3906/952" magazine, this mag has a larger plastic floor plate and a 9-rd capacity. This locks in place properly and functions fully as intended in any Model 639 so don't hesitate to buy these if shopping for extra magazines.
The whole line of 2nd Gens only lasted from 1983 to about 1989, I wouldn't use the terms "rare" or "scarce" but it would be totally accurate to say that S&W produced much, much less 2nd Gen pistols than either 1st Gen or 3rd Gen and it's honestly not even close.
The 639 is a fantastic snapshot of an interesting time in American handgunning as a heavy all-steel, full-size but not high capacity 9mm marketed in an age where almost -ANY- buyer that selected a 9mm was looking for 13-17 rounds in the magazine.