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I am not sure the same coding scheme is used for long arms, but try it and if it sounds right, it might be.
As seen below, there are four possible marks on any Star firearm.
Other regions have different proof houses with different marks and codes.
Every 28 years or so (there are inexplicable gaps) they start a new series, by adding a number to the end of the letter codes.
This practice became most organized, and earned the name of "proofing" with the advent of firearms, when armor plates were proof tested to assure their resistance to (proof against) early gunfire.
Though 100% of armor in those early days was tested, this rapidly changed to batch testing, which is what we have now with most firearms.
At least one reader has a model PD with, apparently, the month and year of manufacture directly stamped on the frame.
Note that proofmarks are not highlighted; this has been emphasized with white crayon before imaging.
A representative sample of each lot of weapons is tested with a proofing cartridge, which has vastly higher pressure than the normal specifications.