One such Machin is that of this 6p value available from the 1995 Congress at Hayling Island "overprinted" for the occasion when Francis Kiddle signed the "Roll
of Distinguished Philatelists".
These were produced by Victor Short, and several can be found illustrated in "100 years of the Philatelic Congress of Great Britain, 1909-2009" by Colin Searle, which is available via the ABPS.
It is obviously very easy for anybody to manufacture their own overprints on any stamp. This is done mostly to gain publicity for stamp dealers, stamp associations and I will not rule out although I have no positive proof of this ,occasionally to hoodwink serious collectors. Most have been manufactured for gifting as souvenirs to collectors.
Apparently it is not against the law to do this, but (I have read) " it does render the overprinted stamps invalid for postage"
Early in the 1980s the late Ken Gibson (who at that time ran the Lundy Philatelic Bureau on behalf of the Landmark Trust) gave out as souvenirs of certain values of the then current Machin definitives overprinted.
Although it is not illeagal, Ken did not get away with it lightly. As a result of these unauthorised overprints (and many others on Lundy stamps) his contract was terminated. These shown above are on phosphorous paper; it is said "that other values may also exist"
Other overprints in private hands were issued to those who attended the Scarborough: Philatelic Congress of GB in 1998.
Various stamps were overprinted "PCGB SCARBOROUGH 1998"in different formats. These were only available to delegates attending the congress. I believe that similar items were produced for other Congresses also.
I remember when we first started this blog (many moons ago now) I reported on a private 1st class Machin upside down overprint from Australia. Sorry , you will have to plough through the archives to find it.
We also showed other Machins overprinted Somalia. Again go back through the archives for this one. Another post reported on a government Machin which included pictures of a 2d overprint also another early post which shows some rather nice Boots labels.
What ever next: A Machin Mania overprint perhaps :-)