It was made by Champion and it's a great little unit. My friend recently bought one just like it. The pea-green paint is a dead giveaway as well - that was Champion's old paint color. I'll check with my friend and see if he can get a model number off of his. Grainger also sold a lot of these pumps under their "Dayton" or "Speedaire" brand names, and there is a single-cylinder variant of it as well.
Here is the pump (they don't make this exact same model any longer) that Grainger sold as a replacement part:
http://www.grainger.com/product/CHAMPION-Air-Compressor-Pump-2Z050
Maybe somebody else can chime in with the Champion model number for your pump - [edit] I'm thinking after doing some more checking that this was a model AV1, BV1, CV1 pump:
http://www.needanownersmanual.com/product_info.php?products_id=690
Here is the modern-day successor to your pump (note the exact same crankcase bearing side cover):
http://www.grainger.com/product/CHAMPION-Air-Compressor-Pump-5FTU2
Here's another thread about the same compressor pump:
http://www.garagejournal.com/forum/showthread.php?t=146088
This site has a lot of repair parts for your pump (that I can identify by sight) - you might call them if you need anything:
https://www.factoryaircompressorpar...ampion-compressor-repair-parts/?orderby=price
As to the unloader on yours, I believe it has an orifice in the check valve fitting right at the tank, on the upstream side of the valve. It leaks a tiny bit of air while the pump is running, but then allows residual pressure to dissipate once it turns off. Crude, simple, no moving parts, inexpensive, but effe
Thanks for the info. I needed it. If the fly wheel is 1-3/4" (single belt track) wide it is the AV1.
Jay