Jim,
I believe it was actually a parkerize finish, but manganese phosphate is close.

fwiw my NOS set is actually a grey phosphate

I've never seen zinc on original 4spd rods (69-71)

Edited, i went back and re-read your question. I had thought you were asking about the rods. not the shifter mech itself. I answered about the , rods.
Shifter mechs are zinc plated as shown in examples below.