Plus 512/SP Calculator

Plus & Sumlock Mechanical Calculators
PLUS 512/SP Special Calculator

Plus model 512/SP

This appears to be a Special calculator with a strange keyboard and operation for an unknown purpose.

It has an abbreviated keyboard (only has keys 1 to 5), so to enter a number greater than 5 in any column two key presses have to be performed in succession in that column which add up to the number required.

The machine illustrated has serial number 512/SP/91.373/Q.

It is a model '512/SP' where the 5 denotes the abbreviated keyboard (keys up to 5), the 12 denotes the 12 columns of keys, and the 'SP' maybe denotes a special purpose version of calculator, with the '/Q' maybe denoting the purpose of the machine.

Made by Bell Punch Co. Ltd. in England.

This machine is unusual and its use is a mystery because:

The keyboard and the display window are split into three equal sections with each section having four columns of keys with the least significant column being a different colour.
All 12 columns operate on base 10, and all carry in the usual way just like any other 12 column decimal calculator. There is no separation between the 3 groups of 4 keys, it carries from one group to the next.
The paint is factory original, taking it apart, for cleaning and repairs, shows that clearly.
it looks like it never had decimal point markers from new since there is no sign in the holes along the front of any screw or peg, and the paint is intact in the holes.

Grateful thanks to Matt Davey for providing the photographs and description of this very unusual machine.
If you have any further information about this model please get in touch.

Rear of the calculator with no sign that the usual circular "PLUS" badge was ever attached in the recess provided.

There are excellent photographs and descriptions of the Plus Adder mechanism at John Wolff's Web Museum site. These are accessed from the Comptometers and Key-Driven Calculators section on that site.

Click here to go to the Vintage Calculators Web Museum