I would think this is a question for the ultra-specialist in Swiss philately. I have tried everywhere to find the answer but as yet to no avail! It concerns the test or trial stamps produced by Karl Bickel for the Swiss PTT. Two of those are well-known and well-documented, but there apparently is a third one which seems to baffle everyone.
When I visited the museumbickel in Switzerland last summer, I was lucky enough to be given the opportunity to research the stamp archive in their depot. In that archive I found various copies of a stamp which looked very much like a test stamp. I asked the archivist but he could not enlighten me any more than assuming it was a rejected test stamp, apparently because the engraving was too fine.
I got a photocopy of the page to take home with me, and started asking around all my knowledgeable friends. This yielded a few more illustrations but not much more detail. But at least I now had a better picture of a single stamp.
And, more importantly I now had an illustration of a folder which included several of these stamps, with annotations of certain stamp denominations.
With all this I dreamt up the following theories:
The style of the stamp is very much like Karl Bickel's more abstract work, so it would be a fairly safe bet to suppose the stamp is his. Though it ain't more than a bet. But check out his abstract engravings
and his abstract stamp essays
and I think you will agree with me.
Then there's the matter of size. During Karl Bickel's stint for the Swiss PTT, two new printing presses were procured: the SSR I Goebel in 1935 and the SSR II Wifag in 1945. Bickel's two known test stamps were produced for these two machines. Now, the SSR II was immediately set up to print larger format stamps, those being of the size of the 1949 definitive set. The SSR I was originally set up to print small format stamps. The mysterious test stamps are in the small format, and will not therefore have been produced for the SSR II.
The test stamps are recess-printed and recess-printing only started with the SSR I so that must have been the printing press for which this stamp was made. But then, we already had a test stamp for that machine, and various test stamp experts have stated it is highly unusual (better still: unknown to them) to have more than one type of test stamp for a single machine.
The supposed theory of it being engraved too finely and therefore discarded doesn't sit well with the folder which has a huge range of copies. What's more, they're printed in the colours used for the 1936 definitive set, so that would suggest a printing well into the production phase of that set.
You see? It's a proper mystery! I've asked the author of the standard work on test stamps (which does not mention this stamp at all); heck, I've even contacted the archivists of the Swiss PTT, but either the silence is deafening or no solution is forthcoming.
So this is my last straw. I'm just hoping that someone somewhere might read this some day and actually know about this stamp and can tell me more/all about it!