Identifying shower valve from pictures

I need help identifying this shower valve. The knob broke, so I made the mistake of lending vice grips to a tenants so they could still operate the valve until the new knob was delivered. Long story short, the tenant broke the stem (argh).

I have attached photos here. The valve is along an exterior wall, and I'm hoping I can identify this and replace a cartridge. Of course, just to make this extra interesting (frustrating), there no shut off except for a whole house shutoff. So, it would be ideal to know the cartridge before removing it.

What did you order a knob for, meaning what brand?

Actually, I use a universal knob Danco 80015. Its unique design allows it to work on a short stem such as this (of course, not with a broken stem).

Update: I had a plumber come out to inspect. He said in 21 years of doing this, he's never seen this kind of valve.

I think it's delta, but best thing to do is remove the guts and inspect

I don't think it's a Delta. I have never seen one like that before.

I think my plumber would have recognized a Delta. He did try to remove the cartridge, though it's in there so tight that he's afraid of damaging the pipes in the wall by pulling any harder. Neither of us could find any sort of retaining mechanism for the cartridge, so it should just release. The plumbing in the wall is CPVC.

Unfortunately, there are no stops on this valve, and he commented the trim plate "looks" smaller than the Delta trim plate.

The present plan is to cut out an access hole and replace the unknown valve with a Delta. If we do this, I'll look for any other markings on the valve itself once it's removed, because I am quite curious myself. Either way, I'll be happier with a conventional Delta valve in the end.

Looks like a valley valve works like the old delta but no ball inside

Upon closer inspection you're right that's not a delta.