Ah, good spotting, I see the same thing in '65: 1613 TA7 for all Darts and Valiants except convertible, 1614 TA7 for convertible. Your 1648 BX7 does show up in '66 for Valiants, while Darts continue to specify 1613 TA7.
can H stand for type of mat? Like Herringbone, Gray plaid etc?