Sorry guys, still confused and want to get it right. Read through pages and still not quite sure.

The trailing arm nuts have a lip on them, and per PET (4) are required.

Click to view attachment

It makes sense that this lip inserts into the trailing arm mount and the link bearing. However, mine will not fit. Initially I thought that my nuts were squished (ouch!) and the lip was deformed. But after filing a little bit away, they still will not fit.

Also per the PET, a washer goes on the mounting ear side. If a washer goes there, the nut lip wont insert into the mounting ear. If the washer is on the inside, the knurled side of the bushing rod wont seat against the mounting ear.

Click to view attachment

After much searching, I may have found new nuts, so I have ordered (4). Interestingly enough, AA sells repos without the lip.

So is this lip really important? Is it only for the outboard side?

I've always seen the nut with the lip on the outer side and not in the trailing arm hole.

That is just one style of a locking nut. The lip will always go on the outside. Any "all metal" lock nut would suffice. The type with a plastic lining would be prone to failure if exposed to extreme heat.

Those are self locking nuts.
The locking feature is on the end with the raised lip.
Its slightly crimped (out of round), so no wonder you couldn't get them to thread on the shaft that way.
The trailing arm pivot shaft has a shoulder the same diameter as the hole in the mount. That's what keeps it centered.
If those nuts aren't available new, you can substitute nylocks.

Thanks guys!!