It's all a bit hit and miss. To begin with all the Ks were sold with bare aluminium timing chain, cam & crankcase covers, bellhousing, gearbox, swingarm and final drive.
Then, around the late eighties/early nineties, black bellhousings, gearboxes, swingarms and final drives began to appear on some models. This was about the same time that the K1 was seen with powder coated (yellow) items.
When BMW later released the K1 in Classic Black it had bare aluminium.
The follow-on K1100 models were all powder coated silver or painted black (the black is a paint and not very heavy either).
I think that the first model to have the black parts was the K75S but not all K75S have them. I think that it depends on the colour of the fairing and tank as to whether it had black or bare. To confuse things further, the original gold coloured K100LT had black timing chain cover, cam and crank covers yet had bare aluminium from the bellhousing back.
I'm afraid it's not quite as simple as pointing to a model year as both finishes remained for sale side-by-side for quite a while.