Mazda 1990-00 oil pumps have a hole in them for the dipstick on FWD applications. There's a blind plug pressed into this hole on Miata applications. If you don't install the blind plug on a new pump, you will have a non-pressurized oil leak at the front of the engine that will drive you nuts.
You could probably get the old plug out of your old pump, but for less than the price of a cup of coffee - why risk it? This is a new OE plug.