The 1965 Mustang Body Assembly Manual (AM0006) pg 86 (the page number is blurred on my copy - ref N5-8930) shows the addition of the spare wheel plate, C5ZB-1424-A, and "hook" bolt, C5AB-1448-B, both with an effective date of 10-9-64.
This does not really establish an absolute date, but a Ford engineering change to an assembly line procedure. We know that Ford used up existing items in many cases.
Jim