Allan Oliver has posted this reply to my original post "Post Office Training Stamps": Many thanks ALLAN
First to introduce my self. My Name is Allan and along with a GB collection from pre-stamp to circa 1951 there are a number of side-line collections. The items form a joint collection with my father and are housed in two locations, London and Shropshire. After reading this item on Training School Stamps I thought a few notes may be of use for collectors. Machin stamps with overprints have been around in bulk since at least 1980 when we started forming our reference collection.
The influx since circa 2000 seems to have happened when a number of training schools were being closed followed some years later by this material being offered for sale. As a brief run down of the history - the training schools started in London in 1920 and soon spread to other large cities and towns, followed by smaller areas also having training schools established. At this time the stamps used were those in general use and these were overprinted with black bars.
In the reign of King George 5 every basic stamp is known, both definitives and commemoratives with the exception of the 1925 wembley set, the £1 black of the PUC and the £1 green of the seahorses. From King Edward 8 through to Queen Elizabeth 2 every stamp is known with a training school overprint of some form, be it black bars, (horizontal or vertical), application by pen, hand-stamped SCHOOL SPECIMEN or similar etc. To return to the decimal Machin issues, those with printed bars are the general supply sent to the training offices on request.
The idea of the overprint, as it has always been, is so that the training schools do not need to account for the stamps and also so that any in the public domain can not be used to pay for postage. The overprinting by pen was done at offices where the supply required for a course could not be order in time and these were transferred from the main office, overprinted by hand and used. This problem had been about since the reign of King George 6 when a number of offices were supplied with a hand stamp reading SCHOOL SPECIMEN which they used to overprint additional required items. This was used for a limited number of stamps at each operation but remained in used for a very long time, explaining why items appear well into the 1960's and maybe later.
In short Machins with printed black bars are not fakes and in our experience, turn up more often than those with the pen type overprint. There are also a number of ways in which this material came onto the market legally and more details can be supplied should they be required
Allan has offered to follow this up with a more detailed post on Machin Training Stamps, which I look forward to very much. He also has a complete history of training stamps published on the web which he will leave us a a link to.