Belgium 1883 50 C Mh Imperforate

Hello everyone

I believe this one is quite rare error. Any idea about It's actual value?

I wonder if this is a cut corner from a postcard?

It would be twice an international letter rate for the time. It might be a faded example. If so, it would be from the right column. It might be a reprints from some philatelic souvenir.

As LastDodo has a strong following in Belgium and some of its catalogue manager are Belgian, I would have expected to find the postal stationery there. I do not see it in their stamps section. But that site is an utter mess, so it, still, could be listed in some obscure category.