I sent him a e-mail this morning. Tried to be tactful:

"I'm going to pass on the rifle. I had some concerns with the stock markings as they are different than any I've seen. I showed the photos to an expert on these rifles and he concluded that the stock is a replacement stock of some sort and not original to the rifle. The markings are inconsistent with any known Germanicon examples.

The rifle would be great to a shooter but not so much to a collector and I'm not a shooter.

Thanks for the offer though it is nice seeing these old warhorses."

If I get an "interesting" reply, I'll let you know.