Could be.....

The dropped zero hypothesis would fit with both my book and what you describe. The book does not give cumulative totals, or starting and ending numbers for each type, only the number of cars made for each model year (letter type) which, as you describe, starts in August and runs through to June or July of the following year. I believe there was time off for assembly line workers during the summer, while the plant was reconfigured for the next model year.

There were also "and-a-half" model years, for example, the difference between a 72 140 and a 72 1/2 was that the latter had white lettering printed on the fake wood dash next to each control knob, describing its function (pull to turn on wipers, twist to squirt, and so on) and, if the car in question was a 145, the 72 1/2 model had rear shoulder harnesses and the 72 did not. (I've owned both) On 71 142Es, there was a difference in the location of the fuel pressure regulator, depending if it was a 71 or 71 1/2. I'm sure there are plenty of other "half-year" differences that I am not aware of.

The 68 upholstery material is distinctive---very different from the pebble grain used in the later cars, both in color and texture. It is a medium brown, rather than plain tan, and smooth on the outer parts of the seating surfaces, with a fine grained woven texture on the pleated center areas.