I don't know the implementation date, but I think the style without the "staked" end is an early type, 1964 and 1965, and the "staked" end is for later 65 and 66, and maybe into 67.
The first picture shows both the RH and LH type with the plates attached. I got no idea where they came from or what years. Info only.
The 2nd and 3rd (-02-01 & -02-02) show the staked ends but with two different "stakes", I assume from different suppliers.
The 4th and 5th (-03-01 & -03-02) show the plain ends.
Note: Replacement seat to body mounting studs are available from AMK, B-11662, but they are shorter than the originals, 1 7/16 vs 1 3/4 in.
Jim