I will concur that standards tended to drive the technology back then. No specification of how to meet them, just "meet them".
Everybody tended to go to catalysts (i.e., after0-treatment) to get the affected vehicles into compliance.
The tech at the local dealership showed me a graphic of where "heat" was/would be under the hood of a cat-equipped vehicle. Quite a bit, according to that graphic!
What I also understood was that catalytic converter-equipped vehicles were banned from many federal lands due to the heat of the exhaust system possibly causing fires from grass contact with the hot exhaust system. Whether this was from "fear" or "fact", I'm not sure as cat-equipped vehicles were purchased several years later, by the affected federal agencies, it seemed. The Chrysler exhaust systems had an environmental heat shield that hung below the converter itself, as my '80 Newport had. The bottom of the GM-pellet converters had a layer of heat-reducing material in the bottom of their case, which probably served the same function, by observation.
Chrysler did have THREE engines in '75 and '76 that had no converters, from the factory. The 318 2bbl, 360HO, and 400HO. What might be termed "fleet engines" which would be installed into A/B/C-type platforms for law enforcement vehicles, typically. I don't believe they were certified for CA or High Altitude registrations, though, just normal "Federal" emissions areas? These cars, along with the ELB cars, would have a "Non Catalyst" decal on the lh front door, rather than "Catalyst". They were different colors with both having black printing. One was gold and the other one silver?
So, check the VIN for the engine code and also the FSM for the emissions regulations for the particular vehicle. PLUS the emissions compliance decal under the hood. There should be some letter abbreviations on that decal to represent the emission control items on the vehicle.
As for "older vehicles", be sure to check to see what your region checks and considers at the time of vehicle inspection/registrations.
Pictures?
CBODY67