master cylinder?

That is wierd and getting beyond my ken. The 1970 MC's listed on NAPA are different from the later disc brakes MC's. I do see 1 listing for the 1970 with a larger rear reservior that is listed for power brakes. But it is the tall version of the MC body. (And online listings aren't 100% reliable.) And who knows if someone put an oddball MC other before you just to get it going.

I would be inclined to replace it then; the tall reservoirs for dics drum provide extra fluid capacity for the front reservior for the caliper pistons as they move out with the pad wear. And the extra height helps keep the rear reservoir fluid capacity the same as in earlier, shorter drum/drum MC's, despite the rear reservoir's length being reduced.