Well... ain’t this interesting. I’d have lost this bet.
The White-Line recoil pad by Fray-Mershon was available starting in late 30’s and up into the 40’s at least. Fray-Mershon was bought by Pachmeyer in 1955. Which means the white line no slip buttplate could possibly date to late 40’s. Fray-Mershon even had a trademark on White Line in 1937.
http://www.lcsmith.org/faq/recoilpad.html
