The SP1 stamp is the "Savage Proof" stamp that merely indicates it was proof tested at the factory.
The circled 52 stamp should be an assembler/inspector stamp that's simply indicates which employee assembled the firearm.
The squared 3 is a new one for me so I'll wait and see what Mad Dog has to say on that one. My Model 30L from 1960 has no such marking, just the SP stamp, an assembler's stamp, and one other encircled one right by the rib that is to faint to make out.
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