Take a look at this...

[scosgt]

Specifically the "US" markings.

http://www.gunbroker.com/Auction/ViewItem.aspx?Item=499541152

[jim-d]

Receiver looks a little "beat", but the stamping is pristene ???

[scosgt]

EXACTLY. Stampings are straight, perfectly struck, and clean. Way too deep for hand struck originals 100 years old.

[keith smart]

Looks like SARCO