John : Good find, and great idea to try changing the "6" to a "3" - makes sense.
When I Posted that I had gone through all of my MPC's with "no luck", it was before Scott had Posted the illustration that showed the Group Numbers ; yet even then, the "6C523" wouldn't have appeared. Also, I apparently didn't look closely enough at the PS Kits in the 1965-72 MPC, which might have at least provided a clue. In any case, great "digging" and perseverance on your part.
A couple of interesting observations, which might explain how I "missed it" in the 1965-72 MPC. In going back to it to try to find out what the symbol next to the two "Clip" listings in the C8OZ Parts list stood for, my first look did not show the listings that you Posted, for either the C7OZ or the C8OZ Kits. As I kept looking, I found that for some reason, there are two sets of "Kit Parts" listings ; one set on pages 2 through 7 (C5DZ - D5DZ) and pages 8 through 32 (C4DZ-D2ZZ), all in Section 36.1, all pages numbered consecutively, and all dated May 1975. However, in looking a little farther, at the beginning of the "PS Kits" in the Text section, there is a "Page Number Index", which notes that pages 2-7 are "PS Linkage" Kits, and the rest of the pages are the pumps, hoses, etc. I must have not looked at the Index initially, and was thus looking at the Linkage Kits, which is why I didn't come across any Straps or fasteners.
It was in the second section of listings where I found the pages you Posted. Interestingly, the Strap and Clip (retainer) shows up in the C7OZ and C8OZ listings, but not in the C7ZZ and C8ZZ listings.
Also of interest is the symbol by the two fasteners shown in the C8OZ listing (C7AA-3E523-A and C8AA-3E523-A) notes "Optional". This symbol does not show up in the C7OZ listing, and in addition, the "Clip" (retainer/fastener) is shown as C7OA-3E523-A. Also, both the C7OZ and C8OZ listing show the quantity required for both the Straps and Clips as "2" (possibly upper and lower Straps ?).
So now we have Part Numbers for both C7 and C8 Straps and 3 different fasteners (Clips), none of which, for some reason, are listed in the C7ZZ or C8ZZ KIts, as well as not knowing the differences between each.
Just another case of "the more one knows, the less one knows".
Thanks again for "digging" these out.
Bob