I'd call it a modified screw head, if you mean something like a D-shape instead of a full round head shape. I see them in combination with a 'fluted' retaining ring that's tightened to a desired position (rather than brute force tightness), then the D-shape head (which often has two holes or dimples to allow slight rotating into position that prevents the fluted ring from moving). It seems British call many wrenches 'spanners' but in US English a spanner seems to more commonly refer to a 2-point wrench. You can use stiff sharp-point tweezers lacking a proper tool.
I have also seen such hardware in lenses that have set screws to set the infinity focus registration (like old D- or C-mount) lenses.
But lacking a proper replacement screw with the altered head, it seems you either have to find a parts lens (slim chance, and possibly wasteful), or what Bernard & Olivier said...use an alternative anti-rotation means.
If you 'popped one off', sounds like you lost the screw head, not just one corner of the flatted portion. If I lost the entire screw head, I would choose some kind of rigid adhesive, and clean any lubricant, if present, from the area you want adhesive to stick. Nail polish is good, and easier to find than specialty adhesives made as anti-tampering indicators. Use a pin, needle or dental pick to apply as small a drop of it as you can manage. I have used epoxy, waiting until it is getting to the end of its working time (stiff enough to not run into other places it will cause problems, but still pliable enough you can transfer some). I've also used solvent-based glues like "Goop" that get pretty hard but peel off or dissolve with solvent later if needed. Hot glue might not be tough enough.
