Interesting. I had been led to believe that there was no difference in interior dimensions on the early birds like mine.
It's a common misunderstanding. The history of the Cherokee line is convoluted.
The first Cherokee to be certified was the PA-28-160 (1961), with full-sized permanent bench seat in back, plus baggage area and baggage door. Externally it was identical to your -140. A few months later the PA-28-150 was certified, identical to the -160 with slightly less performance and useful load, but able to use cheaper 80-octane fuel. At the end of 1962 came the restyled "Cherokee B" line, including the -150, -160 and the new PA-28-180, and offering wheel fairings and an alternator for the first time.
A year later the PA-28-235 was introduced. The interiors of the -150, -160, -180 and -235 were all identical to each other. In fact, the engineering prototype airframes of the -160, -180, and -235 were all the same airplane, N2800W, modified over and over again.
Meanwhile, Piper desperately needed a trainer to equip its dealer and flight school network. Cessna was fast building a loyal customer base with its loss-leader 150, which kinda looked like its larger corporate siblings. But Piper's only new two-seat trainer in 1961-63 was the high-wing, tube-and-fabric Colt, derived from the old Tri-Pacer, and which looked nothing like Piper's more profitable current models.
They were working on a nifty little low-wing, two-seat trainer, the PA-29 Papoose, but its new composite airframe structure was not ready for prime time -- or even direct sunlight, as it turned out. The PA-29 project was abandoned, leaving Piper still without a "modern" trainer.
In 1964 as a stopgap move Piper took the Cherokee 150, removed the back seat, moved the aft cabin bulkhead forward one station (making the baggage door unnecessary), de-rated the engine to 140 hp (2450 rpm), lowered the gross weight to 1950 lb, and called it the PA-28-140 Cherokee 140. To compete with the 100 hp Cessna 150's operating economy, Piper quoted a 50% "instructional cruise" power setting.
A year later, Piper pencil-whipped the 140's power and weight specs to equal the Cherokee 150 (150 hp at 2700 rpm, 2150 lb gross weight), and offered the "2+2 Family Cruiser" package.
Meanwhile, the "Cherokee C" came out in mid 1964, encompassing the PA-28-150, -160 and -180. This was mainly a cosmetic change, with the new 235-inspired fiberglass cowl and a slightly larger instrument panel; while the Cherokee 140 kept the original panel and metal cowl.
By 1967 the PA-28-140 was far outselling the -150 and -160. The -140 had a little less cabin room in back and a little less style, but identical performance to the -150, at 77% of the -150's purchase price. So the -150 and -160 were discontinued.
