Jaydawg-
That sure looks like factory checkering to me, the problem lies in whether it officially left the factory that way….I know of many Ruger Newport rifles from the past that were checkered after purchase, or unofficially, or on the weekend/during lunch, etc. I have even personally held a Ruger 44 Carbine with exceptional walnut stock and hand checkered AT THE FACTORY in the style of a Winchester. I have seen beautifully stocked 10/22’s with many different checkering designs done by the ladies at the factory for numerous employees. The guys who ran and worked in the wood shop have the best rifle collections!!
I would request an updated letter specifically regarding the factory checkering, and suggest you send some pictures along. Sadly, there is no one left at the factory who understands what was what from the 50’s-80’s, so if the paperwork doesn’t specify it left as a SPC model, it wont letter as such, even if it was done prior to leaving.