what?

I am putting my money on that being a GM master cylinder. It should have numbers on it. You can go to any GM dealer and get a new one. The newer ones have a slanted top on the reservoirs which fit perfectly under the hood. Did this one hit the hood a tiny bit when the hood was closed?
Take that with you to the dealer. My invoice from a few years ago shows that as being PART # 18030337, and the parts dept could not find what model GM it went to.

Another option is a Willwood model. Offered by people like Summit Racing. They offer GM style masters with different piston diameters. I op'ed for the 1" 1/16 dia piston. Moves more fluid with shorter stroke, but increases peddle pressure. I think they also have 1" and 7/8 so you can "tune" the system. Cost is about $110. The wilwood models are cast Alum so you save some weight and ditch the rusty cast iron look.

Thanks for all the advise and part numbers. Oddly enough, the parts store couldn't find a similar master cylinder with the plastic reservoir. We searched all of the early years, both truck and car applications and must have brought out a dozen different ones. I finally settled on one for a 1968 camaro which lines, bore and height seemed to match up. It fit and seems to be working fine. Thanks again, as usual, you guys always come through!