At first sight it seems easy to explain - the reel of stamps was mounted in the machine the wrong way - but that actually wouldn't produce this invert unless the whole reel of stamps was wound the wrong way, because the feed would present the back of the strip to the printer.
The enclosed letter explained: A software fault in the NCR Self-Service Kiosk, which occurred bizarrely only at Sydenham BO, caused the stamps to be to be inverted on a normally-mounted reel of stamps. On a collectors set of 6, the 1st class stamp was printed first, followed by the 1st Large, Euro 20/World 10, etc. Apparently despite this the stamps are still usable as printed, so the Branch Manager was told to continue to use the machine (presumably because of the Christmas post pressures), and it took 8 engineer visits (yes, eight) to put this right.
My correspondent, it seems, asked if other stamps could also be loaded into the machine just as I would have - I would have wanted an example of each. [Apologies for poor editing of this paragraph earlier, it's almost impossible to do on a pad as there is no mobile interface and the web-editor version is clunky beyond belief - Google used to provide an App, but no longer supports it!]
And so the MA16 Poppy, MA14 Machin, and Ladybirds are also currently available on eBay with this error - which is much more than 'an example of each'.
UPDATE 12 December
I'm reminded that when a roll of stamps is replaced in a PandG or SSK machine, either due to a fault in the machine or when new designs are issued, the old roll is often dropped into a storage area within the machine - I've seen that myself and I and my customers have benefited from it. If the 'part roll' - let's say they are Machins - has sufficient stamps on it to make it worthwhile using it it may be reloaded when the temporary (say Winter) stamps are removed. If there are too few to use they ought to be destroyed of course.
Back in 2014 new rates of Europe 60g and Worldwide 60g were introduced when rates changed. Many of these were able to be produced on stamps which were thought to be long out of use. We reported the following from around the country: All three water life, all three farm animals, union flag, Machin, spring flowers, robin, and lastly Birds 2, 3 and 4. Obviously these were not all from the same branch but several different branches had some very old stock, and boosted their turnover as a result. Most branches only had two designs active, but could produce others if asked.
This is unlikely to happen again, not in the near future. But errors can occur - albeit not many on NCR SSK machines. But all of us have the opportunity to pop in to SSK branches and buy a stamp - if it turns out there is an error, then buy more. Of course if you not are a regular visitor you may not be able to persuade the staff to add something different, but if you cultivate your sources by being a regular visitor and finding friendly staff, then you can achieve more - a fact that applies to ordinary post office branches and buying booklets or other stamps as well.
And don't begrudge others their good fortune and the work they put into sourcing different stamps. Without these people I wouldn't have anywhere near as much to report!
UPDATE 15 December
We've had a very interesting comment from Paul who is a software designer with NCR - see 15 December comment. Paul writes:
"I'm the software designer and lead developer at NCR for the Post Office self-service machines. My first inclination is usually to assume that our software is at fault until proven otherwise, but in this case it seems quite unlikely. It is possible to generate this kind of reversal if the media have been removed from the roll and fed into the printer the wrong way, which I suppose could happen with a left-over roll."
My response to this, which doesn't actually explain how the problem occurred:
"Invert the image and the stamps will be correct, and the printing will be in the correct order, ie the first stamp to be dispensed in a Collector's Strip is the 1st class (stamp number 43 on the Machin). The second stamp is the 1st Large - stamp number 44, and so on to the Worldwide 100g, stamp number 48.
So the printing is in the right order on upright stamps, but the print is inverted appearing at the right of the stamp instead of the left. The printing on each stamp *must* have been printed with the datsstring printed first, and the top line of '1st class' printed last."
As an aid to understanding what has happened here is the Machin strip up the right way - see right. The top stamp is number 43, the next is 44, then 45, and so on.