Its the washer on the end of the pivot shaft that would go between it and the suspension ear
Its #8 in this diagram

Even though it is shown in PET I have never seen one and have removed many trailing arms. There is really no room for a washer on the end of the pivot shaft. The end of the nut is nearly flush with the end of the shaft.

Well I just removed mine today-there is a rubber washer the size of the trailing arm surface at the end of the trailing arm. The original one is in pretty tuff shape. I am sure this arm has never been off this car by the looks of the condition of the under coat that is on the nuts bolts and other parts.

Also without this the arm surface would be hard against the suspension ear rubbing metal on metal as the arm pivots with the suspension movement.

Could you post a picture

I will tonight

I will tonight

Philip, (this isn't meant to sound like I'm know-it-all) that's actually not how the stock trailing arm setup works. The pivot shaft is butted directly up to the suspension ear and does not rotate. Though the arm itself rotates, the rubber bushing (if it's still intact) is actually sandwiched between the arm and the suspension console ear. The rubber is stationary and simply flexes with the travel of the arm up and down.

Like Bruce, I've never seen a washer on a stock car. I do know that the polybronze rear trailing arm setup does require a washer inboard of the suspension ear. On TC's yellow banana car it's obvious that the washer is required to allow the nut to bottom out. But their design is completely different.

I just went out to take a closer look , the old one kind of looks like what you are talking about. Bit I measured both of them and my new one is 31.2mm from each end , the ones I just took out were 31.4 , 2mm longer here are comparison pictures.

So I can see that these new ones would be too long with rubber washers and do not have an indentation for a rubber gaskets and the old ones if they were removed would not the have the whole surface flat, the pivot shaft would stick out.
Anyway I guess I can mount this as is and move on. So does any one want my old ones?

NO rubber washer in between the suspension ear and the arm. The bushing inside the arm is supposed to cover to the end of the shaft. #8 in the diagram is most likely the metal washer on the OUTSIDE of the suspension ear under the nut

Well that is interesting. Except the ones I took off had no washer on the outside.do I need to source one?

I'm not sure it is necessary. I've stripped down some cars that had a washer and some that didn't. FWIW it's a lock washer, not a flat one. Also I'm ASSUMING that is what is being shown in the diagram as I have probably dropped 20+ suspensions and have NEVER encountered a rubber washer

Yeah, I agree with Scotty. Perhaps that rubber washer was there to take up some perceived slack by someone who aligned it before?

I believe the stock nut is a lock-nut. The kind that needs periodic replacement

The correct washer is flat. The nut is a locking style.
There was never a rubber washer inboard of the console from the factory.