I recently purchased used three 1DMIIIs and a 1DsMIII.
I went into EOS Utility then clicked on Camera Settings/Remote Shooting. I wanted to change the owners name. At the top of the pop up screen is a file number. Here's what came up for each camera:
1DMIII serial number xxx148 is file number.........6299
1DMIII serial number xxx571 is file number.........7135
1DMIII serial number xxx995 is file number.........7940
1DsMIII serial number xxx293 is file number.......3817