Great Britain Envelope Stamp With Image On Reverse

The top-left image shows a heavily inked Great Britain half-penny envelope stamp cut square from 1892 and a more typical copy on the right.



Shown below is the reverse side of these cut squares. As you can see, the copy on the left contains a mirror image. How common or uncommon is this?

I believe this is common. At least here in the States these envelopes are embossed which leaves an imprint on the other side. That it is colored is less common - that may be an error?

Peter

It's common enough, of minor interest and of little value. Either the ink somehow got onto the underside embossing die or the normal ink soaked through. Seeing how heavily the face is printed/overinked on that one, I think it's the latter reason.

Maybe because printing embossed stamp stationery creates more spoils than sheet stamp printing, inspection isn't as rigorous. In any case, there are always tons more varieties and errors for stationery in general.

Not uncommon but nice to have.
Here is one of mine ... a 2d Blue with date plugs 13 10 91 and postmarked 29 FE 92 (leap year)

Thanks for the insights.