IBM produced a small number of disk cartridges that had a precisely recorded signal on specific tracks that would be used by the CE to align all disk drive heads. This serves as a reference standard for alignment, a standardized measuring point. The cartridge had a red cover, to visually distinguish this special cartridge from all the ordinary ones.
|Red cover marking a CE cartridge|
|Official CE cartridge, underside of cart at cooling air door|
|Looks quite clean in the pictures|