A friend sent me a link to this very interesting thread on m.net discussing the differences in the 1.8 BP head gaskets over the years. http://forum.miata.net/vb/showthread.php?t=350182

There are at least 4 different head gaskets:
- 94-00
- 99-00
- 2001+
- MSM


The differences are the holes for the coolant flow between the head and the block. They control the coolant distribution across the cylinders. When the BP engine was adapted to the Miata the coolant exit was moved to the front of the engine.
My guess is that Mazda found that changing the exit resulted in a less than ideal coolant distribution with the FWD gasket and came up with new revisions of the gasket on the NB,NBFL and MSM. The 94-00 gasket is the same as on the 1.8 FWD engines (323f,GTX and GT-R) according to the Mazda Parts Catalog, part number BP26-10-271
Here is a pic of the 2001+ and the 94-00 (top):



Since I'm going to use the M-tuned coolant reroute with the water exit on the back of the head like the FWD engines have I will use the same gasket. It is the only gasket designed for a reroute type coolant exit.