The Model M industrial keyboards were the first PC-compatible Model Ms available, originally shipping with the IBM 7531 Industrial PC. Compared to the more famous and common IBM Enhanced Keyboard, IBM industrial keyboards feature a grey casing designed to hide the dirt and discolouration expected to be inflicted upon the device within an industrial environment. However, the internal assembly is unchanged. Thanks to their colour and relative rarity, these are amongst the most expensive Model Ms to purchase today.