There have been 5 oil pumps over the Miata's lifecycle.
1. The oil pump for the Short Nose Crank 1.6 motor in cars from 1990 to early/mid-1991. Here is a page from the Factory Service Manual illustrating installation.
2. The oil pump for the Big Nose Crank 1.6 motor in cars from 1991 to 1993. The difference between this and #1 is that the diameter of the hole for the crankshaft is larger to fit over the snout of the BNC motor crankshaft. Because of the larger hole for the larger crankshaft, the oil seal is larger diameter also. Both of these oil pump attach to the engine block using a paper gasket.
Here is a pic from the Factory Service Manual showing how the 91-93 oil pump mounts.
3.1. The mountain face of the oil pump where it mates to the engine block has channels machined into it and now uses RTV sealant instead of a gasket - like the oil pan.
3.2. The outlet of the oil pump where it meets the oil channel in the engine block takes an o-ring.
Here is a comparison of the 1.6 vs 1.8:
Here is the parts diagram from the Factory Service Manual for this oil pump. Notice the difference...
4. The oil pump for the NB chassis cars from 1999-2000. This is a different part number compared to others and has been superceded with the newer oil pump.
5. The oil pump for the NB2 chassis cars from 2001-2005. In 2001, with the addition of VVT, Mazda changed to an oil pump with a slightly higher volume. This oil pump is slightly thicker gears and corresponding case to fit the gears. This oil pump also takes an o-ring and uses RTV instead of a gasket.
Here is the kicker, this later pump is also about $100 cheaper than the 1.6 oil pump... Compare prices here for yourself:
I bought the 1.6 BNC oil pump from Mazda Motorsports for ~$250. Had I known, I would have saved myself $100 and bought the later pump. Fun stuff...