The number after the dash is the engineering change. It signifies some change in the basic design of the gun from the previous version.
It generally has a different meaning for each model.
A pinned barrel has a small pin in the frame near where the barrel enters the frame. This pin served to secure the barrel and prevent it from turning out. The pin was dropped around 1982ish (different for each model) when Smith & Wesson began using a "crush fit" to secure their barrels.
The recessed means that the rear of the cylinder is cut so that the rims of the cartridges lie flush with the rear of the cylinder instead of resting on it. If you were to look at a recessed gun from the side, you couldn't see the cases in the cylinder, while you can in a non recessed gun.
Recessing the chambers was done only on the magnums and the .22s, and was, like the barrel pin, dropped 1982ish.
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