Okay. Lot's of research, long story, found the answer, don't like it.
- Overall design of the Nissan B13: one of the things that always impressed me about this vehicle is that it seems to have been designed with the DIY home mechanic in mind. Components are accessible and tasks can typically be accomplished using only 2 different wrench sizes, and maybe a screwdriver and pliers. When I got under there to flush the tranny, I found a drain plug for the tranny.
- Tranny filter vs tranny screen: What? Yes. Some trannies have a filter that you replace when you change the fluid, other trannies have a screen (metal mesh) that you clean when you change the fluid. Filters are usually clipped on, screens are usually bolted on. This RL04F03A tranny has a screen.
- Other people found nuts in their tranny pan: this post from a Miata forum talks about their tranny having a few bolts holding the screen onto the valve body via some threaded holes, but there's this 1 bolt that goes through the valve body and can only be tightened by a nut on the upper side of the valve body. It's just not feasible to get a finger in there to get the bolt started into the nut, you need to remove the valve body.
- But that's a Miata! This is a Nissan! Well, I was a bit surprised to find so many dadgum bolts holding a flimsy little filter-looking thing that turned out to be a screen. I went at it and removed the bolts, and, half way around, started noticing the bolts weren't all the same. Great. Just GREAT. There were 8 long ones, 2 short ones and 2 medium ones. The 2 medium ones flanked one of the short ones and the other short one is on the opposite side, I figured I could remember all that. Besides, it was too late to do the cardboard thing as most of the bolts were out. When it was time to reassemble, it turned out there were 2 kinds of long ones, 7 with threads all the way up, and 1 with threads near the end only. Aw crap. That's the one that needs the nut.
- Really? You need to remove the valve body? Well, if you want to take the screen off and put it back on, yes you should remove the valve body. Because of the risk of introducing dust, dirt and other particulates into the transmission by removing the valve body, many sources recommend leaving the screen in place and just changing the fluid. That's why there's a drain plug.
- Great. So how did the nut come lose and what do you do about it? Well, if you don't know there's a nut up there, you won't go looking for it. That last bolt, the one that needs the nut, won't thread tight, but you may not notice, since it goes through so much metal before getting to the other side and there's still plenty of tranny fluid dripping down the hole to make a "fluid lock" and make the bolt seem a bit tight. Fire up the car and the fluid will swish (that's a technical term) that nut around until the magnets grab it. That's what magnets are in the tranny pan for (metal shavings and things that come loose). That nut has been in there since the very first time the car was started up after the last tranny service. The tranny's been performing fine, it didn't miss that nut or it's associated bolt not being threaded in tight, it won't miss it going forward.
Long story short. On this tranny, don't even bother taking the pan off. Drain the fluid using the drain plug, then refill. I always wondered why my fancier other vehicles don't have a tranny drain plug. It's because they have filters, they WANT you to take the pan off so you may as well change the filter because it's just there. I don't want to generalize, but I would expect vehicles with a screen (not a filter) would have a tranny drain plug to discourage you from getting your dirty little fingers in there when you don't really need to.
And just for posterity, I'll link to 2 YouTube videos I've found about people tearing down this specific tranny. Not a single nut to be found, and the screen was installed off camera while the valve body was out of sight. Don't you just love it when you find a video with a long enough runtime to make you think it's complete, detailed and exhaustive only to find it skips right over the part you really need?
1997 Nissan Primeria SR20DE with RL04F03A
1997 Nissan Sentra GA16DE with RL04F03A