Editing Comptometer (section)

===Model E===
[[Image:Comptometer model E.png|thumb|right|<div align="center">Model E "Controlled-Key" Comptometer with a white release key (1914)</div>]]

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.

Each key was supported by a metal plate that wrapped around it on three sides. This created a support that was used to block the key when the error detection mechanism was activated. The blocking mechanism was moved inside the box in the following models.