Oh, the rectangular hoods for the 50/1.8 and 50/1.9 are perfectly functional on the 50/1.5 lens. I've noticed no problem.
I suppose that they could cause some subtle vignetting, since the 50/1.5 shade is funky in using a Series VII filter on a 40mm shade. Or maybe a slight loss of speed wide open. But I'm not using this lens for speed outside.
From mocking the 50/1.5 hood up with a 40mm Series VI adapter and VI to VII step-up ring (and the hood from the 85/2.0), I can tell that the "correct" hood is rather of a viewfinder eater. Of course, I use a IV-SB2, where the viewfinder window is close to the lens, the V, VI, and 7 probably are less bothered by this hood. (After all, people find the 50/1.4 lens acceptable on them.)