...Turnover to next model year is done in August so your '72 is really a '73.
I think the DMV title dating process back in those days was probably more calendar based than model year based.
I think you had it correct early on about emissions. It wasn't
only the fuel-return lines though (as those were not on all Broncos anyway from what I remember), it was
EVERYTHING ELSE related to emissions under the hood.
DMV has no say in the matter. No law states that after August they must register all new vehicles in the next model year. The OE tells
them when it starts and nobody else. And every manufacturer does not have the same literal cutoff date matching the others. The calendar-based cutoff is Jan 1st of the model year in question.
Of course, they make mistakes often enough, but they don't dictate the year model to the manufacturer. The safety and emissions standards do.
So like I said, I think you had it right with emissions.
Maybe Ford's suppliers could not come up with enough cold-air filter housings. Or EGR intake manifolds, or the valves themselves. Or the carburetors with the proper metering and bowl vents to go to the charcoal canister. Or any of a number of changes.
I'm sure you've seen it, but anyone that looks under the hood of a registered '72 and then a registered '73 (that has not been butchered over the years) would not recognize it as the same vehicle unless they were a gearhead and used to seen such changes take place.
As far as EB's are concerned, it was probably the single largest change under the hood of any model year. There were more and more thingies and hoses each year after, but the changes for '73 were monumental!
It's when the Broncos and F150's had to more closely match the standards of passenger cars, than of the heavier "light" trucks. They waffled back and forth for years, but in '73 the big changes took place.
So again, I think you had it with emissions changes.
Paul