Need to wire a new light switch onto old wiring. All red, one is in comm, other 2 are in same hole but not marked. New socket is L1 and L?

A cheap $10 multimeter comes in handy in situations like this. I had to rewire the front porch, living room plugins, lights and all the switches but they had it jumped so much you couldn't just throw it together. Either take a picture of the light switch so you can follow their same pattern or get a multimeter to check for hot and ground and trace the wires where they all go. I'd suggest the multimeter since you are just jumping to a new switch. Plus your have it on hand when you have more small projects.

As long as you follow the same configuration for the new switch then you shouldn't have any problem. New ones tend to be C(om), L1, L2. Older ones could be L1, L2, L3. (Com would be L1)