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Looks like the indented part of a friction plate guide used in the LSD carrier. Overall width on a new piece like that is 15.5mm, but where yours appears might've broken, I measure about 12.5mm, so pretty high confidence this is it. It installs on the tabs of the friction plates. You can see it in the hole in the pic.

Regarding filtering...I needed to do something similar a few months back. I used paper coffee filters in a funnel. Took a while to get all the oil through but worked fine. Rebuild's not too bad. Amayama is a good route for parts. I also have LSD parts around. If you have an older unit that happens to have tabbed spring plates (see my post for examples), then I have a set of unused friction guides for you.

I was trying to figure that out, too. I can see that if it shifted a little maybe it could be sheared off by the carrier over time, but if it had sheared off, that piece would've been full width. Because it's short and the cut looks somewhat clean (can't see the angles), I could guess that it could fatigue off...but how incredibly unlikely it'd be to fatigue in two spots. The piece also can't exactly be ripped off...it's a pretty dense chunk of metal. All said, I'm baffled. Donors shouldn't be too tough to find depending on how many yards you have nearby. HG43 and HG46 just refer to the ratio; the 3rd members can be swapped without issue, just need to be mindful your truck and the donor have disc brakes (should because SE?), otherwise you may need to split the carrier to add/remove a thrust block inside (discs didn't use the thrust blocks, drums did) Could it lead to catastrophic failure? Potentially, yes, because if the remainder of that guide is in the carrier, the lack of that piece on it will allow it to slide out of the hole in the carrier. The piece alone will destroy things if it's caught in the gear mesh, but then also the friction plates lack the piece and may get bashed up. You wouldn't be able to gut the carrier because the clutch pieces provide the necessary tension on the side gears to keep them engaged to the spider gears. Amayama is a good source for parts still, but I'll be surprised if they don't cancel the order for the 1.6mm pieces...they should be discontinued (I've tried ordering them a few times). The difference is just thickness and how it contributes to the stack height (thicker stack, higher break-away torque on the unit). I have a write-up here about all that stuff, but it can also serve as a rebuild guide. You'll be better off just finding a donor and having extra parts. I think I may even have enough parts to build up a 4.375 carrier, too, if you're unable to find something local.

I have never cracked an RE4R01A open, much less rebuilt one. But I've seen enough questions about what's different between them that I figured it was about time I went down a service manual rabbit hole on this topic. The short version: VG and VQ RE4R01As are not the same. Even within each group, there are multiple variants shown. I have no idea which variant(s) the rebuild kits are meant to cover, but I would start by working out how many clutches yours has, and how many clutches the rebuild kit includes. The long version: The number of drive/driven clutches in each pack is laid out in the SDS at the end of the AT section of the service manual, so I looked at a few of those to compare them. Each manual shows multiple "codes" of transmission. The code is the last five digits of the part number for the transmission assembly. Go to NissanPartsDeal, punch in your VIN (or year/model/options), look up an automatic transmission assembly, and check the last five digits of its PN against the codes below. If it's not there, check the SDS in the '96 manual specifically, or see if the listing shows a superseded part number, and check the code for that one. I expected the first five digits to be the same for each part. They are not. Looking up which vehicle each code fits is therefore more difficult than it has any right to be, which is why I only bothered for the first four. '95 W/D21, codes 49X02 (31020-49X02, 2WD), 49X03 (31020-49X03, also 2WD), 45X60 (31020-45X60, VG/2WD), 45X72 (31000-45X72, VG/4WD) Reverse: 2 drive, 2 driven High: 4 drive and 4 driven for the first two codes, 5 and 5 for the last two Forward: 5 and 5 for the first two, 7 and 7 for the last two Overrun: 3 and 5 Low/reverse brake: 6 and 6 '97 R50, codes 44X82 or 44X20 Reverse: 2 drive, 2 driven High: 5 drive, 5 driven Forward: 6 drive, 6 driven Overrun: 3 drive, 5 driven Low/reverse brake: 7 drive, 7 driven '02 WD22, codes 43X64, 4EX01, 4EX06, 4EX67, 4EX68, 4FX07. First three are VG33E only, last three are VG33ER only. Reverse: 2 drive, 2 driven High: 5 drive for all, 5 driven VG33E, 6 driven VG33ER Forward: 6 and 6 on the first one, 7 and 7 on the second third and fourth, 8 and 8 on the last two Overrun: 3 and 5 Low/reverse brake: 7 and 7 VG33E, 8 and 8 VG33ER '03 R50, codes 4EX74, 4EX75, or 4EX76 Reverse: 3 drive, 3 driven. High: 5 drive, 6 driven. Forward: 4EX74 has 7 drive and 7 driven, the other two have 8 drive and 8 driven Overrun: 3 drive, 3 driven Low/reverse brake: 8 drive, 8 driven Again, I would look up what's in the kit vs what the code says you need. Hopefully they include enough bits to rebuild any of the variants over the years it's meant to fit. I don't know if anything else changed enough between the two to potentially cause you problems. Again, I'm not a transmission guy. If you're on the FB page, see if you can find what Elim Naelcm posted about his transmission. IIRC he Frankenstein'd a VQ R50 RE4R01A to work in his TD27-powered WD21, so if anybody would know, it's him. I haven't seen him around in a while, though. There was also a guy here who rebuilt a WD21 box, but I don't remember his name, and again I don't think I've seen it in quite a while. I also vaguely remember seeing an R50 trans rebuild on Youtube once--you might look that up and see if they mention which kit they used.