Well, both the part number and the illustration in the factory parts catalog changed. If the Tstat didn't change, they went to an awful lot of work to create a mistake. I also believe the original thermostat was a 180F unit.
Check out TSB T-65-12, which talks about an alternate "pellet type" (so unsleeved since all the sleeved ones were aneroid) thermostat, 82C, for the TR4.
But I could certainly believe that someone, perhaps even the dealer, installed an sleeved 160 thermostat to make the engine run cooler. After all, "everyone knew" that was the right thermostat for a TR.