I've found interesting things under buttplates, and forearms both. Under one old rifle I found a dealer's paper tag reading, "Metropolitan Goldstock's Jeweler's and Sporting Goods. Schenectady, NY." With a 5 digit phone number on it also.
On another I found the gun maker's name stamped into the inside of the buttplate. On this same gun I found "#100 TESTED" under the forearm on the bottom barrel flat. And on still another I found the owner's name stamped on the bottom barrel flat under the forearm.