Believe that is an Assembling Screw. Numrich is out. Other than finding an organ donor rifle, you are probably looking at having a machinst make one. You can check with Jack First parts- but they do not have an on-line catalogue.
When I lived down South , I was always going to "ACE-HARDWARE" .!
Down where I lived , in the nuts and bolts section , I was able to find just about any metric or SAE ,fastener known to man .! I have not had to try that up north yet , but down "Yonder' that store always amazed me.!
Who knows , maybe you'll get off easy with a little blue-ing..?
That's nice to know, that link at Homestead Parts.
Now that I've used up about a whole day making a holder, crowning a Rossi bolt like a tooth capped w/new threads, and I didn't even bother with the hole.
And after all that, I was gonna give it to my cousin who hides his guns from his family so he doesn't have to teach them responsibility.
He lost his in the '60's. After I went to all that trouble, I decided to keep it, even if it won't fit my Rossi.