Service fill for the AW4 is only four quarts - even if all you do is pull the drain plug and refill.
There should be only three bolts thru the filter, so the fourth is worrisome. IIRC, the filter bolts are the same size as the short bolts holding the valve body halves together, so there's a major seal that isn't happening. Valve body leaks cause loss of transmission control (even with the electronics, the internal control of any automatic transmission are hydraulic, and no valve body action means no holding gears.) Only park (a mechanical lock) and neutral will work consistently.
The brass-coloured job IS a thrust washer. I don't know which one offhand (there are about five in there,) but it's going missing will also cause trouble. I'd guess it was down there due to material failure - probably a casting void or an unplanned seamline in the casting.
I also concur with replacing the unit with another AW4. It can be rebuilt (I think a master kit with seals, gaskets, and friction will go for about $300,) and it is a relatively simple box to service. If you are anywhere near San Jose, CA I can help you, or Beezil in Chicago, who has been thru a few of these as well.
Apart from failures like this (probably caused by pilot error,) the AW4 is a rock solid slushbox, and is well worth keeping. I put 220K on my 87 before I lost the box, and that was due to losing three bellhousing bolts and an inch of daylight in the middle of the fourth. Even with all the abuse I heaped on it, when I tore it apart for failure analysis the only problem was the first planetary set being cocked in the case from the misalignment of the axes of the crankshaft and the input shaft. The clutches were still nearly new, even after some major towing jobs and a boatload of city and cross-country driving. I rate it better for daily use than anything save the HD Alison boxes and the like.
If you aren't near San Jose or Chicagoland, post a request for help on the OEM tech board and see if anyone near you has been thru an AW4. You don't need a lot of specialised tools, and those you do need can be made with a little creativity and effort (I made the bridge pullers and a device to pull the large snaprings from scrap stock on hand and some threaded rod, probably for less than what a single "special tool" would have cost to buy...)
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