STOP!
That is your reverse detent.
There is a spring in there with a ball down in the bottom.
If you take that bigger fitting out of the trans, I'm pretty sure the ball can fall into the trans, and that will cost you time taking the cover off to fish it out.
Instead take the smaller nut off first, and with a magnet, fish out that ball. After that you're clear to go.
However, that detent rarely leaks.
Almost always the thing that leaks is the reverse switch up above it. When they leak, the oil is almost always going right thru the bakelite, around the perimeter where it is "crimped" together, and/or, right alongside the metal connecting spades.
If you buy new switch, chances are it will leak right from day-one.
Better it is to seal the old one. I use JB weld after meticulous cleanings, both inside and out. You will need compressed air. To clean the inside, you will need to depress the ball and shoot the brake cleaner in beside it. At that time, you can witness where exactly it is leaking on the other side.
JB weld is a lil fussy on this job; it doesn't stick to oil. But IMO, RTV is more fussy, and cleaning a botched RTV-job, for a redo, is not fun, cuz RTV does not stick to cured-RTV.
Good luck!