Well, after a lot of research and internet surfing I was able to find actual evidence of whether or not the rear BMW E82 135i callipers would fit the E85/6/46 chassis, and they do,
but not the M's.
The M's have a different geometry in the trailing arm that prevents the use of these.
the next two photos are good to show the difference between the two
Look the calliper mounting lollipops of the NoN-M, they are shorter, you can see how the holes are tangent to the edge between the two lollipops
Now if you look at the M version, see how the holes protrude further
I was able to find the following photo of someone who had test fit the 135i calliper on an M3, you can see how the calliper is mounted further out radially
So this option has been discarded obviously.
So I have sourced a set of rear 996 Rear Calipers which I have disassembled and cleaned for painting.
