London Postmark Oddity

Here is a minor puzzle found on a 1940's cover from Switzerland to England. There is, at first glance, little about it that is unusual. A scribbled notation near the top is mostly indecipherable though it may have something to do with the puzzle.




Now, here is the puzzle. The London Postmark in the lower centre of the envelope is reversed. I cannot think of anything that would cause this other than an offset from another envelope but I have never run across that before. The clarity and the depth of the strike seems to argue against this.

Shown below is the original reversed image from the envelope and the the mirror image flip.

Any suggestions?

Here it is flipped over and darkened a bit.



Is there any impression that presses through the paper? If not, I would go for the offset idea myself.

Easy enough, if a person and over-inked a cancelling device, to lay the envelope aside and have something else accidentally laid upon it.

I agree that this was caused by offset from an envelope below it made by an extremely consistent postal worker.

No, there is nothing at all on the back of the envelope.

I guess that the offset is the best explanation after all. Why look for something complicated when a simple explanation is available.

Thanks for the input.

Yes, I would join the offset group,

an example I found many years ago from NZ