Greg:
"Into this era the 40" range market was nearly petered out and cost was likely a huge factor when 30" ranges were so much less expensive."
The cost was the killer. In many '40s and '50s houses originally equipped with 40-inchers, including ours, housewives in need of replacement ranges took one look at the price 40-inchers had climbed to, and decided to make do with a 30-inch range. In addition, self-cleaners had become the most desirable ranges, and 40-inchers with self-cleaning were priced the very highest of any range.
Many a late-'60s/early '70s kitchen had a 30-inch range in a space designed for a 40-inch model, with whatever was handy filling in the extra ten-inch space. Ugly, but hey, Milady had a self-cleaning oven!
In our house, a TOL Lady Kenmore 30-inch smooth-top with self-cleaning replaced the 40-inch Frigidaire. I will never forget the ordeal of getting that all-porcelain Frigidaire to the curb at the end of our 200-foot driveway, without a dolly.
Yes, we would all kill for that Frigidaire today, but at the time, nobody cared. And pre-Internet, very, very few people would have been able to come up with the factory-NLA parts it needed.