Fuel pressure regulator

I just realized that the unmarked (by unmarked I mean clearly labeled and ignored) device on the carburetor side of the valve cover is a pressure regulator. The regulator isn't exactly late model and it doesn't look like a Nissan part. And, I doubt that I'll be able to find too much information on it.

Should I keep it or ditch it? If I should keep it, will it fail and how will I know? It's got numbers 1-5 with tick marks in between. I assume that these are psi. Is there any other unit they could be? rods/hogshead, etc. It's set to 2.5 at the moment, which makes me think it is psi, because I've heard 2-4 psi is a good number for fuel pressure. The car has the flat-tops on it at the moment and they are way rich. Should I try to adjust the regulator before the swap? after?

Thanks.