I decided to look into this further.
I dug out The Bronica Manual by William Cheung, copyright 1989. All ETR series lenses are referred to in it as "E" not "MC", though the 75 and 500 are elsewhere referred to as E-II. Tamron's ETR Guide states that "MC" was dropped because multicoating had become widespread by then.
(If "E" equals "MC", what if they were squared?)
Comparing the Tamron specs and the specs in Cheung's book corroborates what wiltw said-the PE are different designs.
Medium Format Cameras, Users Guide to Buying and Shooting, by Peter B. Williams, copyright 2001, says,
"The earliest series were designated MC, and for the most part are very good performers. Two lenses to avoid in the MC line are the 150mm f4 MC and the 75mm MC--the 150mm f4 because it is a poor performer and the 75mm because of mediocre performance and a 58mm filter thread (virtually every other ETR lens has a 62mm filter thread). The 150mm f3.5 MC, which replaced the f4 lens, is a true standout, with quality that matches the latest PE version. The MC and E or E-II lenses are optically the same, except for the 75mm f2.8 E-II which is a vast improvement over the MC version....
...All PE lenses are computer designed."
Thanks to wiltw for challenging my misconception.
So stradibarrius, as for the 200/4.5 MC, I'd not hesitate to get it, but if you want to be sure you're getting the latest design, go for the PE version.