If you haven't seen it, there's some good reading on Skywagon model-year changes at
skywagons.com.
Here's the paragraph on the debut of the Ag Carryall (introduced with the side door in 1970, and with the addition of spray gear in 1972):
"Ag carryall" option introduced with external 151 gallon chemical tank, removable spray booms with 30 nozzles, wind driven spray system, windshield wire cutters, and vertical stabilizer cable deflector. If used as a sprayer, the engine TBO was 1200 hours.
And here's another on 1977 model year changes:
1977 Model A185F - Serial numbers 3154 through 3458 (only 304 built)
Improvements and New Features
McCauley tail wheel and stronger tail wheel gear/spring tube (1 1/8")
Redesigned control wheels, small.
New basic flight instruments in "T" configuration.
Engine - phase II, heavy case.
Incidentally, I've just been reading Cessna: Wings for the World. It suggests that Cessna knew the Ag Carryall was a mediocre spray plane, (and it had already introduced the Agwagon in 1965) but that offering 185's in that configuration allowed the sales department to circumvent embargoes on sales of personal aircraft to certain South American countries. The author surmises that in most cases the spray gear was ripped off as soon as the planes were successfully exported.