OLD tub faucet replacement HELP!

This thing is so old that it's crumbling when I put the allen wrench in the tiny hole underneath. My problem is that when I put the allen wrench in, I cant find anything to unscrew. I've tried every size wrench that will fit in the hold and still nothing. I can't see in there either. We're renting the house from my fiance's mom and she swears we need a plumber but it's just an old faucet, I should be able to handle that.... with some help

If there is no Allen, then it just unthreads from a pipe nipple.

Thanks! Can I use a set screw faucet that I have to replace the old screw on one or should I go buy a new screw on faucet?

If it unscrews there is no set screw. You would need to get a threaded diverter spout.