First to illustrate the two different versions of both the bracket or receiver attached to the rear most fold down panel and the bumper on the L shaped fold down section
Early version of the rear stop/bracket
Later - pocket style version
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Here is what I found on an earlier fastback (Oct 64 San Jose) Adjustable rubber bumper
Later pocket style rear fold down bumper - Not rotated correctly but does give us a better look at its unusual shape
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The following is what I found going through my pictures and don't relate to the pictures above.
With that foundation it was difficult for me to find many pictures as it seems that allot of people take pictures with the fold downs in the up or don't include any pictures of the rear interior
Highest VIN with the earlier version of the bracket I have pictures of is 5R09C185xx (Projected Build Date Feb 65)
Can't tell for sure with this angle but here is a picture of the bumper area from 5R09K2141xx (PBD end of April 65). Can't tell for sure but may be the earlier version. You make the call but to me it appears to be the circular version. A little difficult to make out - black bumper against a black seat.
Unfortunately can't use Shelby pictures to help with this detail