Early history of GPO Stamp Vending Machines
The following extract is taken from: Stamps of Great Britain Issued in Rolls and the Machines which Use them by C.M. Langston and H.C. Corless, published 1960. Edited for tense and extra information by Steve Knight, 2006.
The insertion of a coin first lifts a coin plate, then, when pressed, the coin enters the machine lifting the "driving weight". This is connected to a rack and pinion, which ultimately drives the stamp feeding mechanism;
At this stage the coin falls on to the coin guide and rolls past the testing device for ''misformed and abnormal" coins. When the coin has been accepted by the machine, it falls into a chute and thence to a balance arm. This acts as another testing device, as it only operates if the coin is the correct weight. This arm on being fully operated allows the escapement mechanism to rotate and permit the "driving weight", lifted when the coin entered, to draw the feed wheel sufficiently to feed one stamp through to the stamp aperture. Unaccepted coins are rejected and can be picked up in the cup below the stamp aperture.
The roll of stamps is placed on a spindle and held there by a metal arm. The stamps are then fed under a fixed guide bar, over a second guide bar and then under a bar which is pivoted to a lever. When the roll is finished this lever drops causing the "EMPTY" or in later years, "NOT IN USE" plate to drop over the coin hole. The stamps are now fed over a drum containing fixed pins, spaced at a distance of a stamp and fixed into the horizontal perforations, and at every fourth perforation at each side.
The stamp roll and dispensing mechanism are enclosed in a cork lined metal cover. (Not on all machines Ed) Fixed to this, near to the roll of stamps, is a dehydrator containing cobalt chloride crystals in a perforated container, the action of which is to absorb dampness from the atmosphere. When the crystals are dry they are a blue colour, but when they have absorbed their maximum amount of moisture they are pink. They can be dried out in an oven and used again.
The last style of machines have a lever which allows the "Not in use" plate to drop if the roll should break or the machine jam for any reason. Earlier-machines did not have this and the local Postmaster had to insert the plate.
Vending Machine Testing: the labels used
When setting up, adjusting and testing the automatic vending machines, Post Office Engineers made use of rolls of dummy stamps (Labels). There were numerous different types of these dummy stamps produced between about 1910 and 1935, for both Post Office machines and private ones. The main groups are: (all are perforated)
(a) Plain or coloured paper, with or without watermark, gummed or ungummed.
(b) As last but containing some design such as the Head of Thomas Richard Harrison instead of plain paper.
(c) The "Poached Egg" type issued by the Post Office in 1935/6. These were perforated 14 x 15. watermarked multiple "GVR and Crown" and gummed. Examples of this label are known postally used, having served as a half-penny stamp through the mail. It has been suggested that after testing a machine, a dummy roll was left in by accident, so that the dummies were then vended and used as stamps.
(d) White paper with a thin green frame line and green saltire. Unwatermarked but gummed,
(e) Green paper with a white, gummed back. Watermarked "Harrison & Sons Ltd., London" in script.
(f) Plain white paper, unwatermarked, gummed and perforated.
(g) Plain white paper, watermark Multiple GviR & Crown, gummed and perforated.
(h) For Testing Purposes only stamps, issue in 1938, watermarked (various reigns) gummed and perforated
(i) With the advent of the Type F booklet dispensing machine, the For Testing Purposes Only stamps were issued in folded booklets containing a pane of four stamps.
The rest of the booklet is then devoted to a detailed listing of the various rolls that were issued.