Plus & Sumlock Mechanical Calculators
Plus Adder

Plus Adder

Plus Adder

Decimal, key-driven ("Comptometer") type, with abbreviated keyboard (only has keys 1 to 5). To enter a number greater than 5 two keys have to be pressed in succession in that column.
The lever on the right is for clearing the total.
The 4 and 5 keys in the left-most column are used for subtraction using the "nines-complement" method.

This example does not have a model letter stamped on the manufacturer's plate underneath. If you have any information about the model letters of these early Plus Adders please get in touch.

Made in England.

This is one of the first Bell Punch calculators, which were based on the Petters design that Bell Punch bought from that company in 1936. Some of the patents awarded to Petters are listed on the bottom of the machine.
The case is unusual in being made of Bakelite. The mechanism of these machines operates on the downwards stroke of the keys and there is no interlocking between the keys to prevent more than one key in a column being inadvertently pressed at the same time.

Photographs of this model have been kindly supplied by Wilfried Denz who has a website featuring old calculators at


Close up of the plate on the bottom without a model type displayed.

Click here to go to the Vintage Calculators Web Museum

Text & photographs copyright © 2002 - 2017 Nigel Tout, except where noted otherwise.