You are in one of the "nebulous" areas of Ford Mustang documentation. The 67 Mustang Body Assembly Manual does not show that piece because it was installed prior to the rear Fastback seat being installed. The 75 Ford Car Parts books (aka MPC) is where the "nebulous" comes in. The part is shown in a drawing in the Body Illustration Section 610, page 13 as 601A68 which come back in the Body Section 600 page 2 as Ford part number C7ZZ-63601A68-A, "Plate, Rear Seat Back Catch Striker". The drawing shows only a single screw, 52352-S (U-293) for installation (that's typical). The Body Section does have more attaching installation, 2 each 52352-S8 for each side, plus the one you want - 53100-S8, 8-15X1/2 Oval Head, Phillips drive, 1 each per side, no other information given. Now for the good part - Ford did not service that screw, it's shown as obsolete. We are now exactly where the reason that your post was written.
I do not have a 67 Fastback available to remove that screw to verify its dimensions or its peculiarities, for example - head diameter and tip type.
In the interim, I would use another 52352-S8 screw in place of 53100-S8 - same threads, head type (diameter?) and length. I base this one factor - wherever this was assembled, they would prefer to have on less item to buy, inventory and install.
In the Body Section 600 page 2, this part, C7ZZ-63601A68-A, is shown with a quantity of 1. As there are two latches, the quantity should be 2.
Per Body Section 600 page 2, the above applies to 67-71 Mustang Fastbacks with folding seats.
Jim