Side cover leak

Jus saying, not criticizing;
in the 50 plus years that I have been running A833s, I have never experienced a cover-gasket leak. Nor have I ever glued one on.
Won't say it can't happen tho.
However, I have NEVER had an NSS that didn't leak, sooner or later.
The cure for that, if you need to know; Is to take it down, thoroughly clean it out, both inside and out with brake-cleaner and compressed air, then epoxy the bakelite to the body, and to the pins, squeezing the epoxy into the crimp-joints as much as is possible.
RTV will not be long-term successful. The only thing that has worked for me, is JB Weld epoxy. Let it dry according to the instructions, usually 24 hours.
If you look at your NSS you will notice that the case has been milled to receive it. This leaves a little shelf right next to the body of the switch. If the switch is leaking, there will be oil in this "cavity" . From there it will wick downwards to the gasket-line and from there, forward along it, to the lowest point, and a little uphill again, making it look for all the world, that the gasket is leaking.
BUT, with the car stopped, over time, the oil below the cavity, will almost all drain to the lowest point and you may miss the very thin trail.
Your tell-tale, is seeing oil on the shelf under the NSS, from which it is uphill over the edge so it remains there. Tissue paper swiped across the case below the NSS will also find the evidence.

BTW, you can actually PROOVE exactly where the trans is leaking, by pressurizing it to in the range of 2 to 4 psi, thru a modified filler plug that has been drilled and tapped for a suitable hose-barb. To do this, I drain the oil out first, so that only air will exit the case, and
The vent on top of the tail, has to sort of be sealed.
Try very hard to not exceed 4psi, as IDK how much pressure the front seal can withstand, and you sure don't want that one to blow! I use my LD tester with the regulator turned way down.

If air seems to be escaping from in front of the front U-joint, then there is a chance that your yoke has a metal plate there like the one that is on the front of your od cluster-pin except larger. Sometimes, those have a tiny hole drilled in them, for what reason I cannot say. If I find that hole, I plug it, cuz Dextron will seep outta there and make a mess.

Again, I'm not criticizing, just ragging on the NSS, which has fooled me in the past.