It's definitely an old Delco regulator and just like what MCI, Setra, & Dina used to use!
How do I know this? I have several just like it on the shelf in the shop! I was much surprised that the '97 Dina we have uses the exact same one that the old MCI & Setra's that ran the old gear driven 50DN's ! But we found out it's true that they were all 3 the same just different part #'s for the different makes!
Oh yeah the little screw in the center is the adjustment screw for adjusting the voltage up or down! (used to be when Greyhound would call us out in the middle of the night that was the first thing the guy on the other end of the phone would want us to do is to "turn that screw 2 times one way, and if that didn't do it turn it back 4 times the other way, and if that don't fix it tow it to Nashville or Memphis (which ever was closer) and they'll have to work on it in the morning!"