Rows of postal workers sat at consuls in front of keyboards like this processing mail by “keying in” numbers related to each letter’s address. They could process letters at rates of more than 60 letters per minute.
Multi-Position Letter Sorting Machine Keyboard
MPLSM operators used 20-key keyboards to type in two or three numbers from the ZIP code, so the machine could then sort the mail into one of 277 or more bins representing sections of the country. These operators sorted mail at rates of 60 letters per minute.