It is with great trepidation that I even have to create this post.

If you've been following my M38 project you know I just finished up a D-18 transfer case rebuild. What you don't know (until now) is that I have a leftover piece....never a good thing.  I am asking this question with an overwhelming fear that the answer will result in having to tear the TC apart again. At least it's not back in the vehicle yet. I followed Rick Stiver's rebuild guide and don't find any mention of this thing. It came with the Walck's "Bearings, Seals, Gasket & Small Parts Kit 1- 1/8" Shaft 1945-1953 Dana 18" but I do not see it in the stock picture of this kit on their website nor do I see it in any pics of other supplier's kits. It is similar in shape to the output shaft thrust washer but that's not what it is. The kit included one of those too and I used it. This piece is much thicker and almost looks "home made" as it's not exactly symmetrical nor are the cuts real even.

Any ideas?

The thrust washer shown in the link you provided has been accounted for and installed correctly. From what I can tell there is no other part that looks like this in a D-18. I wasn't even going to post all this but I figured it'll be a week before I install the TC back in the Jeep so might as well get it resolved for sure.

Here's the parts kit. You can see the thrust washer and this mystery (duplicate?) part in the upper right.

Having it be a duplicate would be a welcomed resolution. The more I ask about this the more it seems that's what it is. Maybe Walck's version of a practical joke? "Ha, let's throw in an extra thrust washer to really cause someone to freak out...."

I had mentioned in my M38 Build thread about tearing into the freshly rebuilt tranny to search for a snap ring that I thought I had forgotten about. There apparently was an extra one of those in the T90 small parts kit too. Same as now, at least I hope it was extra...

Are you sure those aren't the parts your supposed to find under the bench when your cleaning up after the re-install?  LOL

I hope you don't have tear it down again.
Gun

I just called Walck's (which I always feel uncomfortable doing for some reason) and they seem pretty sure it's a duplicate thrust washer that should not have been in the kit. If anyone needs a thrust washer, I'll have one in my inventory 

Thanks for going through the trouble of scrounging all that up for me.

You'll never need any of it until the day after you get rid of it.

I trust that was an MGA?.....   Hold on to it