Side marker "lenses", what's the deal?

[Nufsed]

I'll have to re-trace the research on this subject, but the issue was caused by federal regulations and lobbying to change the time frame of their implementation.
The car makers typically would implement coming changes in the model year prior to the requirement coming into effect.
FMVSS108 is the rule that required the marker lamps. At first, the proposed legislation was going to require every *car* to have SML in by Jan '68 which would've been mid-year change. The car makers were already ahead of this and adding the SMLs on cars prior, starting in mid 1967 with car production for the 1968 model year... but (if I recall correctly) the lights weren't going to be required on pickups yet. Then a bit of congress vs bureaucrat back and forth, and the proposed change was amended to lights OR reflectors for the first year (but now including trucks), THEN, 12-31-1969, lights would be required for all vehicles.
GM, in a bit of a scramble to add the trucks to the program (and still be well ahead of its implementation date), went the reflector route on trucks until 12-31-68. They had lights on all models of their cars (as far as I know) already in place for all of the 1968 models.

Another oddity is that *big* trucks (over 80" wide) were required to have the full-on lights from 1-1-1968. It actually started from the big trucks, first, then down to pass cars.

This shot below is from a report fifteen years later, evaluating the effectiveness of the rule (it was very effective)