Help With Austria Scott 295 "Overprint"?

I discovered a strange Austrian perfin stamp (Allgemeine Verkehrs-Bank, Vienna, VB-33, very common). But the holes have nothing to do with my question. It's only about the stamp, more precisely the overprinted/added numbers.

At first sight it's probably a forgery, because of the curves of the numbers, and the line - they seem to be drawn with a felt pen.

But maybe I'm wrong. Does anyone know something about this kind of individual nominal addings on Austrian stamps? Official, forgery, joke ...

It could also be an ink pen of old or an art-type ink pen. I'd guess the latter because the ink has the look of (old) India ink which would not be used in a fountain pen. In any case, the date when this was done is unclear.

Check the lower left part closely. If the cross-out is under the cancel ink (it might not be clear), then we have a puzzle. If the cross-out is over the cancel, then we can call shenanigans.