Plus & Sumlock
Mechanical Calculators


All of the mechanical calculators manufactured by Bell Punch were key-driven machines of the "Comptometer" type.
For more information about the history and families of Plus and Sumlock mechanical calculators go to the page Introduction to Plus & Sumlock Mechanical Calculators.


Plus 509/LD abbreviated-keyboard decimal calculator.

Sumlock 912/S

Bell Punch Sumlock 912/S full-keyboard Sterling Currency calculator.

Petter Mechanical Calculators

The basis of the Bell Punch mechanical calculator models is the mechanism patented by Guy Bazeley Petter, of the famous Petter engineering company.  Bell Punch bought the design and rights of the mechanism:

Bell Punch Mechanical Calculators

The early Petter based models were introduced by Bell Punch in 1939 under the name "Plus Adder":

These are all abbreviated keyboard models, that is they only have keys 1 to 5 in any column.  To add a number higher than 5 then the operator has to press first one key and then another in that column which add up to the required number.

In 1940 a larger, full keyboard (with numbers 1 to 9 in each column), machine under the name "Sumlock" was added to the range, and a separate company, London Computator Corporation, Limited, was incorporated to deal more efficiently with the sales side of the calculators.

In 1950 the name of London Computator Ltd. was changed to Sumlock Limited, and this in turn became Sumlock Comptometer Limited in 1961 when the British operation of Comptometer Corporation was merged.

List of later Plus, Sumlock, and Bell Punch manufactured "Comptometer-type" models.
The first number denotes the maximum number of keys in a column, and the second and third numbers denote the number of columns of keys.  The letter denotes the type of machine.  Some of the machines distributed by London Computator Ltd. have the letters 'LC/' before the model number.  Manually operated machines were often termed "Figureflow".

When the old Sterling Currency (£sd) was superseded by Decimal Currency it was common for old Sterling machines to be converted to Decimal machines and these could retain the old label with its '/S/' Sterling Currency designation.  Sometimes the Sterling to Decimal conversion of a machine was done crudely by someone simply by removing the shillings and pence columns of keys and fitting a blanking plate.

Sales brochure for Plus machines (pdf format).
Sales brochure for Sumlock machines
(pdf format).


Later, top of the range models (most were available with decimal or Sterling currency (£sd) keyboards):


Specialist models:


Note that the model designations of some of these machines, especially the Plus Adder models and the London Computator Corporation models, are not fully understood but from evidence are as listed.  See the Introduction to the Mechanical Calculators for more information.

A Selection of the Mechanical Calculators

Click on a picture for more details and more, bigger, pictures.




The early Plus Adders

Plus Adder
Plus Adder S
Plus Adder D
Plus Adder DA
Plus Adder S
Plus Adder SJ

Plus Adder ST

Plus Adder ST

Calculators distributed by London Computator Corporation

A Bell Punch subsidiary which was renamed Sumlock Ltd. in 1950.

Plus 509/KL/SF
London Computator Corp 509/S

London Computator Corporation Sumlock 912/S

London Computator Corp 912/S

Calculators distributed by Sumlock Ltd. and Sumlock Comptometer Ltd.

A Bell Punch subsidiary, Sumlock Ltd. was renamed Sumlock Comptometer Ltd. in 1961.

Plus 506/D
Plus 506/F
Plus 506/T
Plus 509/C
Plus 509/D
Plus 509/F
Plus 509/F
Plus 509/LD
Plus 509/S
Plus 509/SP
Plus 509/T
Plus 509 Imperial-Weight calculator

Plus 512/S Sterling currency calculator

Plus 512/S
Sumlock 912/F
Sumlock 912/S
Sumlock 912/Y
Sumlock 913/CP/S

The 993 Family

The models of the 993 family were the last new designs of mechanical "Comptometer"-type machines from Bell Punch, and probably the last in the world.
These models have a new design of casing which is more angular.

Below is an advertisement for the 993 family from February 1964:

Sumlock Comptometer 993 family

After 1961 the Bell Punch Co. produced mechanical machines for Comptometer Corporation of the U.S.A.
For example:


Some Sumlock machines can be found labelled 'Comptometer' and with a different model number.
For example:

Further information about these 'Comptometer' models can be found on John Wolff's website at

Specialist Calculators

The Bell Punch Analyser

Bell Punch Analyser

For more information about the history and families of Plus and Sumlock mechanical calculators go to the page Introduction to Plus & Sumlock Mechanical Calculators.

The Bell Punch Company & the Development of the Anita Calculator
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Text & photographs copyright © 2002 - 2022 Nigel Tout, except where noted otherwise.