If you pull the driveshaft out of your Miata, your transmission will develop a small incontinence problem. That output seal is going to start dribbling. Not fast, but enough that it will make a mess and probably pee in your ear at some point. It's even worse if you tip the transmission back, such as if you're doing a clutch change.
This inexpensive plug pops into the end of the trans and seals the hole. It's such a simple thing, but it makes a big difference to the job and your general level of happiness. Every toolbox should have one.
Covers output sizes from 1.300" - 1.550". A stepped design keeps it from falling out. Suitable for all Miata transmissions.