Does the gun actually have a "MOD 20" stamped inside the yoke cut and an "S" prefix on the serial number?
If so, RUN BACK THERE, don't walk, do not pass Go, do not collect $200.
Model-numbered M20s are exceptionally rare guns; this is one of those firearms that gets the vaunted "Too rare to accurately estimate value"
listing in the value guides.
This particular model is one of the few S&W revolvers that's far more rare in model-numbered form than "pre-model" form, and the more later-model features it has, the rarer it is. IIRC S&W did not formally drop the M20 from the catalog until the mid-60s, but production had basically stopped by this time; the gun is rare to the point of near nonexistence after about 1960.
Assuming it's a prewar .38/44 HD in solid "shooter grade" condition, $329 is still a very good price. (The serial number would be prewar without an "S" prefix.) These guns are very hot with collectors now, and it's becoming difficult to find one under $500, even with a refinish.
[Edit to add footnote] Be aware of two warnings regarding this model:
- Prewar S&W revolvers used the so-called "long-action" lockwork with longer hammer travel and a slightly different trigger mechanism than the "short-action" lockwork used in postwar guns. It is difficult- albeit not impossible- to find parts for long-action N frames due to relatively low production numbers.
- Prewar S&Ws tend to be made of relatively soft steel and .38/44s have a very massive cylinder. These factors can cause the cylinder stop notches to wear excessively if the gun is subjected to lots of double-action rapid fire. Consequently, some .38/44s exhibit alarmingly loose lockup- something to look out for if you intend to use the gun as a shooter, especially given the parts situation (see above). Also, this gun is a poor choice if you intend to do lots of DA rapid fire yourself.