Portuguese Newspaper Stamp With Broken Frame

This is Scott P1a from 1876. A sizeable portion of the upper left frame line is missing, along with some of the adjacent design. There's no noticeable damage, either to front or back. Does anyone know whether it's a recognized printing error? Scott says nothing.

Likely just something stuck to the plate, my Afinsa doesn't talk about any varieties as significant as that would be. I wonder if that one was vended or perhaps the company just trimmed them into strips for easier use, the cut perforations on the sides seem to be somewhat parallel, but at an angle.

It does look as though the stamps had been cut apart, and the parallelism of the angle suggests perhaps by a machine rather than scissors. In any case, thanks for responding. I don't own a specialized catalogue for Portugal but was pretty sure that someone on the forum would have info. Something stuck to the plate in printing is a good idea. I'll keep the stamp as an example of that kind of flaw, if not merely as a curiosity.