Try Farrell Industries. I tried a USO for my Cooper 57M and it was out of spec and a hassle with USO customer service. I gave up and went with the <span style="font-weight: bold">mil-spec</span> Farrell.
The US Optics 1913 rail is what I have on my GAP 338. Works great. Search my name and you will see a pic of it. I think it is in the show your rifle thread