the unknown part to help prevent shifting from 5th to reverse on the down shift, part of the rear case. I mentioned that in my trans when I was wondering if the hole should be on the top or the bottom. should be as spring somewhere as well, hopefully still on the bolt. This mounts the side of the case, has a kind of diamond shaped housing held on with two nuts.
The oil gutter is missing completely on yours so you will need. I will dig around and get that part number for you, you can order from courtesy Nissan. Check the rear housing see if there is a long trough that lead all the way back to the tail, it matches up with a hole in the plate that in you pic has nothing installed.
start at about 4:30 for the talk on the oil gutter.
I used some sealant that was advertised as for use in diff and transmissions. Tried not to use too much as it mostly will squeeze out anyway. You will of course want to get new seals all the way around, there are several, o rings too. Mine was leaking a lot at the shifter (inside) which prob is not a big deal as there is no real pressure and its not like its going to start dripping out, but still its easy to get at if you remove the striker rod and use it to knock out the steel plate on the back of the shifter. All this is covered in the video. Also you want to get a new front cover gasket of course. On thing I noticed what while tapping on the counter shaft nut its easy to slightly shift the entire counter shaft forward. I you do this just look at the alignment of the gears they should be fully meshed. even a tiny bit of movement you will see. look at them closely before any work gets started. To be sure I also measured the distance of the front counter shaft bear to front housing to confirm that the shim that was in there was still correct (this is covered in the FSM regarding what shim to use based on the measurement, use a caliper with the depth gauge). Oh Don't lose that shim (its prob stuck with oil to the front housing but will come loose). Looks like a large flat ring. BTW, I am a newb to this stuff as well, don't let it intimidate you, take your time, there are a lot of good folks here with good info that were very helpful to me that have done this before. The main thing I have found about working on any old tech (I do electronics as well) is to take your time, take pictures, document with notes, don't excessively force, and ask lot of questions if you get in a jam. Pay attention to how seals are installed, but generally speaking the seal lip (the open side of the seal) should go TOWARDS the media that your are trying to seal (oil/grease), at least that is what I have gotten after researching it. I know it prob obvious to mechanics but to a newb like me I was not sure.