Plmeek, if the top flat is marked as stated, the only outfit that I can think of to have that inscription would have been Art Ressel's Hawken Shop, which closed in the early '80's... I can't think of any other custom gunmaker that would stamp that on the barrel; it wouldn't be the right thing to do.
There were and still are several "S. HAWKEN, ST. LOUIS" stamps floating around. Some of them apparently came from the Hawken/Gemmer business and some were made up in the 20th century. Doc White still has a stamp that GRRW used to stamp the MHS Jim Bridger Commemorative Hawken rifles.
The font of the stamp on the subject rifle is a little large and is
sans-
serif where the original Hawken stamps (and the better copies) were
serif. I notice the marking on the lock is
sans-serif and may be engraved rather than stamped.
As you know, an unsigned contemporary rifle or smoothbore is usually the builder's first gun made for himself. If he were to hang his shingle out to build rifles to order, he'd either have a set of metal stamps or be learning to engrave. His rifle-work will always be his "business card" in the public eye.
I agree that professional builders would sign their work as a form of advertising or as a "business card" as you say. But a lot of hobbyists, even the really skilled ones, as well as some of the semi-professional builders did not sign their name. Part of the reason was the case the IRS filed against John Bivins and a few others scared a lot of gun builders back then. Some of them quit signing their rifles so the IRS couldn't come after them. Then there were some builders that just didn't feel a need to sign their work.
I was wondering if the R38A could have been for Ressel.
That's hard to tell. Art Ressel would pay some professional builders such as Keith Neubauer, Doug Scott, and Joe Corley to build rifles with his parts set and sell them in his shop. I don't know that Ressel had them marked in any particular way. I have one that has documentation showing it was bought in Art's shop, but there are no markings on the rifle. Based on the finishes on it, I attribute it to Keith Neubauer. I've seen others that Doug Scott made which he signed. The rifle you bought at the auction doesn't have any parts from The Hawken Shop on it, so I don't think it had anything to do with Art Ressel.
It is possible that the "R38A" mark was made by the gun maker, but I don't have a clue as to who it might be or what the mark really stands for.
There were a lot of people building Hawken rifles back in the 1970's and 80's. Some advertised, but most relied on word-of-mouth or only made rifles for themselves and friends.
There's a bunch more of these vintage Hawken rifles that are still in gun safes or closets and will be showing up in auctions, estate sales, pawn shops, etc. in the coming years.