I do not know what the cab originally was. Is there any way to tell that I don't know about??
There might be some kind of serial number on it: look for either a plate/sticker on the back, or something attached to the inside of the cab. If you can find such a thing it will almost certainly name the manufacturer and from there it should be pretty easy to figure out.
You could wreck the control panel, peeling off the CPO and hoping the old CPO is underneath -- but I'm guessing you probably don't want to do that

If you can't figure it out and you're still curious you can post pictures on the klov forums and one of the
crazy old timers experts on there will probably know.
If you do figure it out you should post on here. I, for one, am always curious about these things.