S&W introduced their first K-frame .22, the K-22 Outdoorsman (or K-22 First Model), in in 1931. They manufactured it until 1940. There were about 20,000 of these made. They are easily identified by their prewar hammers and the lack of a barrel rib. These are not Masterpieces in the marketing sense, but they are the direct precursor of the K-22s that got the Masterpiece designation.
In 1940 they introduced the K-22 Masterpiece (K-22 Second Model, or K-22/40), which had a shorter action and an improved adjustable rear sight. It was manufactured for only a year before the company suspended commercial production and converted to production under military contracts. Fewer than 1100 of these were made. Serial numbers are in the high 600,000 range.
After WW2, the company reintroduced the K-22 Masterpiece with some design changes and a new serial number sequence beginning at K101 in late 1946 or early 1947. The K-22 was manufactured continuously under the Masterpiece designation thereafter. In 1957 the company introduced model number designations and the K-22 Masterpiece became more commonly referred to as the Model 17.
The K-prefix in serial numbers identifies the adjustable sight K-frames of whatever caliber, so K-22s and K-38s of any barrel length, as well as the rare K-32s are all numbered together. Model marked guns were introduced around serial number K325000, but that is not a crisp dividing line. Some non-model marked guns may be found with higher numbers, and lower numbers may be found on guns that are model marked. The model marking is found on the frame under the yoke.
A couple of quick indicators: Any five screw guns (easily identified by the presence of a sideplate screw at the top of the plate near the hammer) are NOT model-marked guns. The earliest four screw guns (top sideplate screw not present following a design change) are not model-marked guns and are few in number, because they were made for only a few months before the Model-stamping began.
If you have a five screw gun with a single line address block (MADE IN U.S.A.), it was made before late summer 1948. The four line address block (MARCAS REGISTRADAS, etc.) is found on guns manufactured in late 1948 and afterward.
There were probably about 150000 non-model marked postwar K-22s manufactured. I have only four, so there are presumably lots left for interested parties. They are great revolvers. Good luck in the hunt.