The development of "twolite" headlights was the end of the earlier (fluted) "Type H" headlights.
Twolite allowed the use of "parking lights" which caused changes in the wiring harness (i.e. a third wire to each headlight), the bottom of the steering column switch (extra position for "Park"), and the plugs & appurtences of connection. (Two contacts v. three connecting to the headlight shell.)
My truck (avatar) was March 29, 1929 and it originally was fitted with Type H headlights and a drum tail-light. The former owner in expediency converted everything over to Twolite (his other Model A was a 1931 Roadster.) I have reverted everything "back."
The truck line kind of got the obsoleted "leftovers" from changes in the car line.
Joe K