Which is the original/correct water pump for a May 1971 240Z?

Almost 100% sure that the Nissan OEM pumps used the cast impeller, not the stamped steel. There are extensive discussions out there about which pumps best.

So, I don't know if A is correct but I'd say that B is definitely incorrect. They both look pretty nasty. Are you thinking about using one of them?

You can get a new Aisin WPN-013 water pump. I have no idea whether or not GMB was an OEM supplier, but Aisin is a Japanese based company that is a parts supplier for that country's automotive industry.

I bought a new Aisin water pump off Amazon recently

I don't think either of those water pumps you pictured are "original", but as for "correctness", here's a couple pics from a 9/70 car. Note that this car has A/C and I don't know if that makes any difference, but it's what I got:

Edited June 9, 20251 yr by Captain Obvious

I think both those GMBs are aftermarket and agree that an Aisin with the cast iron impeller will be as close to OE that you'll find today unless you can find one using the part number on this box.