I think the code stamps came with the move to the Stevens plant in Chicopee. The Model 29's switched from a serial number to a code stamp and I don't think GeneB has found any patterning in it.
The assembly code stamps replaced the ser. # markings and they should match, butt, plate, (even scratched in plastic) lower frame tang, and forend . "sometimes" the forend will be short one "digit". Sometimes white crayon/paint stick on butt and plastic plates
The date codes, you stupid SOB.
The date codes were on the barrels of 22's and on the lever boss of 99's - I also think these other codes to be assembly codes, and maybe inspectors codes, and they do not seem to have anything to do with a manufacture date. On the Chicopee made 22 pumps these 'codes' replaced the serial numbers on the front of the receiver and the same codes will be found on the butt stock - that suggests at least one use of this coding system was to keep the fitted parts together - forearms on the 22's are not marked, but they did not require fitting to a specific gun. These codes can consist of a combination of letter's number's and/or symbols with no reasoning for any of them that I can see - some are simple and some are complex, the number of different stamps used also does not seem to have a pattern. I have never seen a comparison of these to anything on a 99 from the same time period so I don't know if there is anything common to both.
Since all this seems to have occurred after the move to the Stevens plants I will mention something Stevens had been doing, starting about 1920, after their purchase by Savage, they used inspection stamps on the barrel & receiver of their guns - I believe each inspector was issued a unique stamp, the stamps used change over time with old ones disappearing as new ones appear and they can be used to roughly date some guns. These stamps were either a letter, a number (one or two digit), or a symbol in a circle - sound similar to the 'code' stamps? - it might be that departments or individual craftsmen were issued a unique stamp, or set of stamps, to mark the parts as they finished them to keep the sets together. I have not yet tried to record these stamps to see if I can find any pattern to them.