If you talked with one of the guys in customer service you'll find they know less about the guns than you do. Their training only goes so far.
The proof mark... and yes they all have a proof mark, is a stamped SP and a number(example: SP7) The SP means "Savage Proof", the number denotes the person who actually did the proof firing. Proofers have a number and a stamp assigned to them.
Another mark usually visible,(example: A21) is the person who function tested the rifle after assembly.
All barrels will have a caliber code stamped somewhere close the nut, and on the bottom of the receiver between the lug and front action screw hole.
Example: M,=.223. Any letter with a circle around it indicates stainless steel. These are stamped by the person installing the barrel.
There are several other marks put on by the assembler, the barrel staightener, and inspectors.
There is a series on vertical numbers on the underside of the tang that are manufacuring code. This tells information about what machine,operator,shift lot of raw materials and other pertinent info to track quality control. This code was implemented about 2008 when Savage went to the new Okuma maching centers.
The serial # is the last thing to be engraved on the gun, as a matter of fact is is the last operation period. It is done fully assembled right before packing.