Speedee Add-A-Matic

Huh?  Was this for people who used base 6?  Where are the keys for 6, 7, 8 and 9?

They're just a waste of space, and more importantly time.  Like its name says, this calculator was built for speed.

Back in the day a "computer" was a person, usually a woman, who'd gone to a trade school to learn to work a calculating machine fast, by touch.  Some used two hands, and because devices like this allowed multiple keys to be pressed at the same time and adding was part of parcel of the keypress, a trained computer could probably add two or three 10 digit numbers per second!  According to autobiographical accounts, the work became automatic to the point where computers could easily girl-chat while racing through columns of figures.

The original machines (Felt & Tarrant Comptometers) used by computers did indeed have 9 keys per column, but computers were trained that moving a hand to reach the 6 key took longer than holding it steady and hitting 3 twice.  Likewise 3-4 was faster than 7, 4-4 beat 8, and 4-5 aced 9.  Given that, some manufactures saved weight and materials by ditching the unneeded keys.  

The Speedee, made in Japan for "CMI Inc, Chadwick" (aka Chadwick Miller Inc of Boston) was a knock-off of the more elegant Contex machine (see some examples at John Wolff's great site).  Its mechanism feels good and works well, and the device is more solid than you might suspect from its plastic construction.