Felt and Tarrant Comptometer Model E

Very few machines of this type were built, but all were made between 1913 and 1915. This was a transition machine that introduced the "Controlled-Key" safeguard, which was part of an error detection mechanism that blocked most of the keyboard if a key was not pressed enough to add its total to the result. When this mechanism was activated, all the columns of keys were blocked except for the one where the error occurred. Therefore the operator could find the column in question, reenter the number and unlock the keyboard by pressing the release key in order to resume the operations.