I too have the 3GM-K on my winchester M1917 stock.
Here is a blurb from someone much more informed than me who looked into it.

!!!Not an official decoding, take it as it is!!!

"....you show a picture of your “W” stock with a “3 GM - K” in a rectangle.**That appears to be the final inspector mark when that rifle was re-built during WWII in USAicon - still searching, but might be a rebuild by contractor “General Motors - Kevinator”.**Still looking…"