thumb screw for pressure release valve

I have a cheap shoebox style compressor, the thumbscrew for the pressure release valve is missing, gremlins came in the night and took it for thier own designs, makes me nervous just thinking what those little buggers might want with the screw. I was wondering is it some kind of special screw or is it just a with a flat head that fits against the hole in the back of the channel that prevents air from leaking?

If someone knows please let me know, I don't feel much like experimenting with something capable of 4500 psi.

Thank you

Dan