You are under a mistaken assumption regarding the Shelby spare tire hold down washer. All 67 Shelby's used a hold down washer . The steel wheel use the larger one and the deluxe wheels used the smaller one.
I should know better than to try 'off the top of my head.'
I thought I'd seen a smaller washer before, but that's why I started this thread.
The MPC lists two replacement washers.
A 4" O.D. 3 tab washer, C5ZZ-1424-A listed for 65/7 models
and a C8MY-1424-A for 68/9 listed for styled steel wheels. Is this the 'no tab' replacement washer?
The 4" flat washer in the first pic has no tabs and I see that Jim's (jwc66k) SS
uses a washer of this style, but flipped so the raised area is 'up'.
Maybe he can report back its diameter.
Richard (67gtasanjose) shows his washer with tape next to it and the witness marks indicate it too is used flipped so the raised area is 'up' as well for his SS wheels (I believe he has those wheels). The width appears to be around 4", but the parallax may be fooling me.