FYI: Optilock rings use 3.5mm 7mm long Socket Head Cap Screws. You won't find these at any local hardware store, and Optilock charges something like $7.00 per screw.... No thanks.
Here's my solution (found on the net...
I ordered a package of 20 from "microfasteners.com;" Total was less than $10.00 including shipping. They have only 8mm long screws, which work just as well.
The head diameter is just a tad too large, so I cut a segment off of an old, right size Allen wrench, mounted one end on a power drill, popped the screw onto the other end, and rigged a Dremel tool with a grinding tip parallel to the screw head. Both the drill and Dremel on, and 5 seconds per screw did the trick (I kept the screw in a small hole, drilled into a piece of hardwood, to prevent it from flying off when grinding.) Cold bluing from Cdn Tire restored the black color to the screw head sides.
These screws are stronger than the originals, and look just as good. Here's the part number, if you want to go this route: SCM3508
Good luck!