Offset Ink Stains On The Top Of The Gum?

Hi Everybody!
I am studying the back of a stamp with some spots of residual ink on the top of the gum. [I see it is on the top by using a video microscope at different angles.]
According to the process of printing them, the gum is applied last.
Does anyone have a similar example, please?
Design:
base 19.25mm
left 22.40
top 19.20
right 22.40 mm
Perf.11 would make it a Scott#552 if flat plate printing. [It feels rotary to me, but, as Don was pointing out the chances of having a Scott#596 in front of me are smaller than being struck by lightning :)]
So, what are your thoughts, please?

I think you talk about the green spots of color on the back of the stamp (in your stamp more at the bottom). this is called "setoff" (earlier also "offset") and shows the stamp being flat printed, so in sheets where the still wet color made a setoff to the next sheet laying on it.

As stamperix notes: some folks use the term 'offset' ink and others use the term 'setoff' ink. I prefer the term 'setoff' since we already use the term offset to describe one of the printing types.
Don

The setoff isn't normally between the gum and the back of the print, please?
On my stamp seems to be on the outside of the gum. [Using magnifications up to 300x at various angles, it looks like the ink is stuck on the outside of the gum.]

Hi Don!
I am comparing this setoff ink with the setoff ink on my stamp identified earlier by John Becker as #498: on that one, the ink is definitely between gum and back of the stamp.
On this one, is on the outside of the gum.
So, that is the reason for asking what is it.

The offset / setoff occurs when the sheets were stacked on top of each other before the ink had dried. I'm pretty sure the 498's were gummed before stacking. Hopefully someone with more knowledge of the printing process will confirm.

From Linns, see number 2.

Offset: 1) A printing process that transfers an inked image from a plate to a roller. The roller then applies the ink to paper. 2) The transfer of part of a stamp design or an overprint from one sheet to the back of another, before the ink has dried (also called set off).

Since this is a used stamp, just how much gum is there? I suppose one way to determine if setoff ink is on top of the gum would be to take a Q-tip,lightly moistened, and see if you can pull up the ink and gum from a small area. If on the gum, it should pull up; if this fails, then its fairly certain the ink adhered to the stamp prior to it being gummed.

aug-stamps, the description of the printing process has been fully described above explaining the set-off on the paper (under the gum), so no need to take that further.
My explanation to your question of ink *on top of the gum* is simply poor storage - whether close to the time of manufacture, in the customer's possession, or even done very recently - where two sheet of finished stamps got lightly stuck together then pulled back apart leaving some of the ink of one sheet stuck to the gum of the other. I am not sure your used stamp would really demonstrate that effect, agreeing with Al, most of the original gum would be absent.

Thanks for the clarification John.

Al thank you for the tip! Some of the ink went off when using a Q-tip but other didn't. So, a case of what John Becker was mentioning above.
Well, I am the happy owner of a Scott#552!
Stallzer, on my #498 the setoff ink is on the paper, not on the gum. [Same moistened Q-tip method.]