The buttplate is from a Mannlicher-Schoenauer 1903, I've installed a few on custom stocks over the years. I used to be able to find them in boxes of miscellaneous parts at gun shows, but that source has seemingly dried up lately. Google pictures of Adolph stocks, he used them pretty often, even had them engraved.
How about instead of googling Aldoph stocks, I go on gunbroker and look at Mannlicher 1903's and look at the ones with good clear pictures of buttplates, here's
one
Mannlicher 1903 #1here's another
Mannlicher 1903 #2What I see are buttplates that have more curvature than the one on that 03 Springfield, they have widow's peaks that are not as pronounced and the screws on the heels are much higher up than on that 03 Springfield, also the workmanship seems to be of much higher quality on the buttplates on the Mannlichers.