I have seen 71 and 72 Bavs with both brushed and painted. Here's a supposedly all original 1971 50,000 mile Bav with painted...
https://www.mecum.com/lots/LS0917-295480/1971-bmw-bavaria-30-s/
Here's an exceptional 72 with brushed...
My OPINION is that generally, the early Bavarias (2800's and early 3.0's) were 2500's but with the "big" motor. Hubcaps, roll up your own windows, no leather, standard suspension, etc. It would make sense that these early Bavs would have painted rear panels. But, here's Fredomatic's beautiful 2500 with a brushed panel...
I'll contact the
E3 Limousine Club and see if they can solve this mystery.
While I'm at it, I'll see if I can find out why my July 1971 production Bav has a brushed heater control/glove box fascia and not wood grain like every other Bavaria I have seen.