Interestingly Canon's own brochure for the 5D MKIII states that the base plate itself is steel to withstand 'the rigors of professional use' (I thought I read in another source it was infact stainless steel) whereas the rest of the body is mag. alloy, I'm not sure if this differs from other canon DSLRs, but I wonder if the different material has caused issues with the black finish not adhering as well compared to the mag. alloy sections- hence the uneven wear pattern with the base plate worst affected?