Sorry, but I believe you are overthinking this.
The CAPEX issue was primarily a philatelic issue and did indeed come out just after the rate change of the letter rate from 13 to 15, so the issue was (in a sense) obsolete. It would have paid the rate for additional ounces on letters beyond the first ounce (15 cents for 1st ounce, 13 cents for additional ounces). Here is a flat postmarked in November 1978 weighing between 7 and 8 ounces sent between two collectors using a full block of 8:
0.15 for 1st ounce
0.13 for each of 7 additional ounces
=1.06 total, with 1.08 paid as a convenience of using an old 4 cent stamp from the 1960s.
Does the CAPEX issue exist on a card? Sure, but it overpays the rate by a few cents. Good luck in your search.