Actually, this stamp could be a double impression. The 2015 Mundiphil catalog; with Afinsa numbers; lists this stamp as having a double impression variety. The catalog for the basic stamp depends on the paper; porcelana is E4.80 with perf 12 1/2. Papel pontinhado catalog is E17.00 with perf 12 1/2. Catalog for the variety is E150.00.
The double impression is not illustrated or further described. However, I have the 10 Reis of this set, from Timor, with a doubled impression. The Reis at the bottom, and a little of the surrounding design, are doubled in the middle of the head. Sorry that I don't have knowledge of scanning, otherwise, I would show the stamp. The point is that the doubling perhaps does not have to show all the way around the head.
Only way to know for sure is to have the stamp reviewed by an expert (I'll post this and search for the link). Up to you if you want to do this, considering the condition of the stamp.
Hy-Brasil is correct about trying to remove old hinges from these colonial embossed issues. I've done removal a few times, but very slowly and carefully.
Try this link for the Portuguese Philatelic Society (of the UK):http://pps.uk.net