IIRC somewhere between WWII and the early '50s for the guns with Tenite stocks. (cellulosic plastic - a material that dates back to the 1930s - created from wood pulp)
Okay, I found this post cached on Google:
From 16ga.com
"According to the guys over on the Savage collectors and Savage shooters boards (admittedly more rifle than shotgun oriented) Savage started putting date codes on their guns in 1949; somewhere on the gun should be an oval stamp with a letter and a number; the number is the inspector's number and the letter started with "A" in '49, one letter per year. None of my 5100's have a date code, so most agree they pre-date 1949.
However the motto over there with regard to Savage is "Never say never" (or maybe it's "Never say always": I forget."
