electric brakes

One of my electric brakes is stuck on my dump trailer. The wheel won't turn at all, it's real - real stuck! Trailer un-plugged of course. I tried rolling it forward and backward but it's kinda soft ground and it skids the wheel. It has a bit of weight on it, and some traction... it's not just...

I'm having a hard time finding information about what kind of brakes these are. I just opened them up and was expecting the more common type... Just got the trailer - not much information about it (title says it's from 1969) Any help/advice would be much appreciated. And outside view before...