Plus & Sumlock Mechanical Calculators
PLUS 509/SP Imperial-Weight Calculator


Plus model 509/SP Imperial-Weight abbreviated "Comptometer".

Imperial weight, 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.

It is a model 509/SP where the 5 denotes the abbreviated keyboard (keys up to 5), the 09 denotes the 9 columns of keys, and the SP may denote a Special version.

238 x 190 x 125 mm (9.4" x 7.5" x 4.9").

Made by Bell Punch Co. Ltd. in England.

The machine illustrated has serial number 509/SP/95.666, and has the rounded style casing which was used before about 1958.

This machine has an abbreviated keyboard - it only has keys 1 to 5 in the standard decades columns, so to enter a number greater than 5 you have to press one key followed by another which add up to the number required.

The keyboard of this machine should be compared with that of the 509/Y weight calculator. This one is odd in that pounds (LBS) can only be entered to the nearest 7 lbs, which is in fact half a "stone" (the old imperial weight measurement which is equivalent to 14 lbs). This allows for an extra column of keys in the tons section. Maybe the SP in the model number of 509/SP designates "Special".

The machine in the photographs displays the total of 164,853 Tons, 13 Hundred-weights, 2 Quarters, and 14 Pounds.


Close up of the keyboard of this imperial-weight calculator.


Fortunately, Britain changed to the metric system during the 1970s and 1980s and this complicated system of units is now virtually obsolete. It does not appear to have been used in modern times in the U.S.A. where weights are customarily given in Pounds (lbs) only.

No electronic calculators are known to have been sold which use this weight system, though a Sumlock-Anita engineer says that he was involved in the modification of some Anita electronic calculators to this system for Britain's National Coal Board in the late 1960s.

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.