The Model 12 was discontinued in 1986.
This gun is known as the Model 12 38 M&P Airweight.
Production of the Model 12 begins in 1957 with the stamping of Model 12 on the (pre-model 12) 38 Military & Police Airweight guns.
It is essentially a lightweight Model 10 .38 M&P model. The 4" barrel was introduced in 1959 (along with unseen 5" and 6" variants). Pre-1966 models will have a flat latch (cylinder release). Models made from 1967 on will have a "D" serial number prefix.
These should be worth upwards of $400 NIB or $300 in Excellent condition and around $250 in very good condition.
Note - these guns were made to a "K" frame size, but until the M12-4 (1984) the frames were .080" narrower. Grips for K-frame guns will not fit properly due to this thickness difference.
IMPORTANT: Check the gun's cylinder to be sure it is steel and not an alloy cylinder if it appears to be an early model without a MODEL 12 (or MOD 12) stamped on the frame behind the cylinder yoke. A magnet can be used for this purpose. If it is alloy, do not fire!