Monaco Fold Error And Odd Reverse Inscription

I have this postage due stamp from Monaco with a fold error in the upper corner. Has any one seen this error before and is it worth anything. Also, when I scanned it it flipped over and I saw it had a strange, small overprint like inscription on the back. Its two capital G's, one backwards and one forewords, like the B's in ABBA, intertwined with a capital B. What is this curious little inscription? Any ideas on that?

I smell a valuable stamp, given the right audience....
that's a beauty.
The mark on the back is an expertisers mark

Search the SCF family forum for "expertiser"

Interesting and nice printing freak. I do not recognize the owner of the mark.

That's niiiiice.

As Rod says, given the right market, and I may add, the right description / title. Some look for these errors (EFOs) (Error, Freaks, Oddities) I know, I don't like the terms either but the EFO is a commonly used term.

The three letter monogram, thus a person's initials for their first (given), middle and last (surname) names is a common way to put your own unique name into a seal / type sort of affair and thus look much more fancy and ornate, Victorian era usage most likely, is, as Rod says, most likely an expertiser's mark, something like an old style certificate for a stamp, although, after a while, these were falsified or forged themselves, either to discredit the expert or to make money from the unwary.

Some collectors, in this era, would also pencil in their own initials or mark on a stamp's back, perhaps as a theft prevention device? so this could possibly be am owner's mark also.

I saw a similar "paper fold" print sell at Ace Auctions
in Perth about 2 years ago, on an Australian issue,
the fold was more pronounced ergo the gap in printing was wide.
It was on a fairly modern issue, and I didn't give it a second look
these errors freaks and ommissions don't capture my attention,
However when it was knocked down in the vicinity of $500
it certainly did.

Is this what is called in French a 'Pli accordeon'?

The EFO price will depend greatly on the width of the fold, the actual stamp, and whether it is a multiple.

In most cases, paper folds that are 1mm or less, single stamp, common stamp... (as is this case), will fetch $10-$50. There are plenty of the $10-$50 paper folds available if you are not picky about which stamp/area.

You really have to have either a clearly >1mm fold, a stamp that was originally a premium stamp, or a multiple... in order to get $100+, unless the buyer is a specialist in that issue/area or the buyer doesn't know better.

The real big money is when the paper fold results in a "color missing" variety. These are often labeled errors, and they DO get a minor catalog number in the Scott listings! These can typically fetch $100s to even >$1000!

Regardless of retail price, your Monaco postage due with the paper fold is a nice EFO to have. Thanks for posting the pic!