I have never seen anything like that either. To determine what the mounting is right now you must remove that 1/2 WHEEL.
Once you can see the shaft end you most likely will see the shaft tapers and also a keyway and key so the wheel cannot rotate without turning the shaft.
Pull the cover plate by removing the 4 screws.
You may need to tap the cover plate off using a piece of wood and a hammer and/or a drift or a puller.
Then you can check around for wheel availability.