I was just looking at Guatemala stamps on
ebay and noticed that there are several counterfeit, forgery or bogus overprints. One "buy it now" item is even asking $350.00 for a very questionable overprint. The alleged Scott #143a has a tete-beche overprint. The Scott #140 stamps with the overprint are not tete-beche.
There were also other stamps listed by Yvert that Scott does not list. Anyone interested in buying Guatemala stamps should beware of any listing that claims to have an unlisted overprint. I highly recommend that you check the Scott catalog over other catalogs. If it is not listed in Scott it is not genuine.
Why do I say this? Well, in Scott's, you will find numerous warnings about forgeries, counterfeits and surreptitiously made overprints. Guatemala expert, Col. Romeo Routhier told me to reject any unlisted overprint. Now, I was not a close friend of the Colonel, but I did visit him a number of times at his apartment in Guatemala City. He researched every Guatemala stamp ever printed and with the International Society of Guatemala Collectors identified all genuine Guatemala overprints up to his passing in the late 1990s.
Col. Routhier was Scott catalog's man in Guatemala and a personal friend of the director of Guatemala's Tipografia Nacional which did the original overprints. Of course, he was not a personal friend of the director in the 1800s and early 1900s but he did learn a lot about genuine overprints.
So if you are considering a Guatemala overprinted stamp, bear in mind that there exists large quantities of forgeries.Some are relatively inexpensive and I even bought a few just to have copies of these fakes. However, I would recommend that high catalog value overprints be expertized.