I suspect this mount came out of the Kesselring gun shop north of Burlington, Washington. Bought a lot of guns from that place many years ago. It's still there. Clarence was a gunsmith and the owner when I lived in the area. Later his son, Ron, took over.
You might send them an email and ask them about it. Here's a link:
info@kesselrings.com The home page is:
http://www.kesselrings.com/They were a real deal old time gun store. Maybe still are. Been a few years since I've been in the store. Every time I walked through the door it cost me money.
DJR