In trying to look a little deeper into the Filler Boards, I went to the 1967 Assembly Manuals. I found the Filler Boards in the Interior Assembly Manual. I have attached a copy of the appropriate page. Note that it is dated 10/27/66, and shows a totally different type of Filler Board than the two previously mentioned. In checking the 1967 Only MPC, there is no 6511229 Part Number shown. However, the 1960-68 MPC does show it as C7ZZ-6511229-A. Interestingly, in the 1968 Interior Assembly Manual, a similar page, only dated 7/27/67 shows this same type of Filler Board.
So, this leads to the question wondering if this type has been observed from any Factory, or was it just the original idea that was scrapped and replaced by the screw in type, and later replaced by the folded type ? If so, why did it still appear in the 1968 Assembly Manual ? Must have had the Assembly line workers scratching their heads.
This may, or may not, lead to a third possibility for this survey.
Bob