1st Edition Stamp on Back of Cards?

[quuador]

@kanesport15 is indeed correct. We don’t know for sure of course, but that’s the most likely scenario.
I also go over both the black and white back 1st edition stamped Base Set cards in my 1st edition stamp variations article.

I’ll quote the relevant part here:

@quuador

Black Back Imprinted stamp:

Now we’re going from the front to the back of the card. On some Thick 1st edition cards you may find a mirrored imprint of a 1st edition symbol right of the Pokéball (imprints from Pokémon 1st edition stamps), in the bottom-right corner (imprints from Trainer 1st edition stamps), or in the top-left corner (imprints from Energy 1st edition stamps).

White Back Imprinted stamp:

Just like the Black Imprinted stamps, it’s also possible to find white imprinted stamps. I’m not sure how these are formed. At first I thought the ink acted similar as a glue, and when the stacks of sheets are separated again, the ink on the back of the card just got removed, exposing the white of the cardboard itself. However, in the white back stamped Pikachu I own I see no evidence of this whatsoever. There is no damage at the stamp when looking at it from multiple angles.
In this picture some more white back stamps can be found on some Energies, owned by @shizzlemetimbers .

Unfortunately not too much is known about them, although it’s not too uncommon to find.
Still a pretty cool misprint imo.

Greetz,
Quuador