Plus & Sumlock Mechanical Calculators
London Computator Corporation 509/KL/SF Sterling Currency Calculator with 'Key-Lock' feature

London Computator Corporation Plus LC/509/KL/SF

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.  It has farthings keys and 'Key-Lock' feature.
The lever on the right is for clearing the total.

It is a model 509KL/SF where the '5' denotes the abbreviated keyboard (keys up to 5), the '09' denotes the 9 columns of keys, the 'SF' denotes a Sterling currency with farthings version, and the 'KL' indicates that it has the 'Key-Lock' feature.

This machine has the red button at top right of the keyboard of the 'Key-Lock' mechanism.  This is a safe operation feature where if a key is not pressed completely, and so does not register its number, then the whole keyboard locks up.  After noting the incorrectly pressed key, the red 'Key-Lock' button is pressed to unlock the keyboard.

This machine has the older mechanism which operates on the return stroke of each key.

The calculator in the photograph above displays the total:
£123,456 16s 10¾d.

230 x 190 x 124 mm (9" x 7.5" x 4.9").

Manufactured in England by London Computator Corporation Limited, 39, St. James's St., London S.W.1.
This was the marketing subsidiary of Bell Punch Company.

The machine illustrated has serial number LC/509/KL/SF/312.


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

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