BarclayG,
The owner of G&R thought "yes" it looks to be a G&R gun but he didn't remember anything about that particular gun. Please keep in mind that they made hundreds of upgrades over 2 or 3 decades. The next step is...you try contacting Bill Mains, and I'll try to contact Ray Swan.

Also, some extreme close-ups of the engraving may help someone to recognize the engraver by cutting technique.