Plus & Sumlock Mechanical Calculators
Plus Adder S

Plus Adder S

Plus Adder S

Sterling currency, key-driven ("Comptometer") type, with abbreviated keyboard (only has keys 1 to 5). To enter a number greater than 5 two or three keys have to be pressed in succession in that column. The lever on the right is for clearing the total.

The photograph above shows a total of £123,456 17s 11d.

230 x 153 x 105 mm.. (9" x  6" x 4.1").

Made in England.

On the bottom, this calculator has lists of patents, and the three U.S. ones were taken out by Guy Bazeley Petter, with rights assigned to Petters, Limited, Yeovil, England (the internal combustion engine manufacturer). One of the patents had been applied for in Great Britain in 1923 and the other two in 1930.
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, see the patents list below.
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.

Cover removed

With cover removed, showing the mechanism.

Plus Adder S

On the rear of the case is  a maker's badge in a recess. This is held in place with adhesive and these badges are often missing.


Close up of the maker's badge on the rear.


The bottom of the machine.

Patents listed on the label.

All of the U.K. patents were applied for by  Guy Bazeley Petter and Petters Limited of Westland Works, Yeovil, Somerset, from whom Bell Punch acquired them.

The application for the first, 208,232, was submitted on September 14th 1922, and the application for the last, 413,373, was submitted on January 20th 1933.

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Text & photographs copyright © 2002 - 2023 Nigel Tout, except where noted otherwise.