On the left is what I think is Portugal 50a. But its blue and I Cannot find it in the catalogue. On the right is 50a with a star punch in the stamp. Any history on the star punch would be helpful.
The star punches were utilized from 1870 to 1893 to invalidate stamps for postage which were used as revenue stamps for payment of telegraph messages.
Ref: Carlos Kullberg Selos de Portugal - Álbum I (1853 / 1910) page 17
(Google translation) "It is interesting to note that the seals (stamps) intended for telegraphic postage, were rendered useless with a five-point star perforation, after they took the mark of the day! Thus, stamps from issues from 1870 to 1893 were rendered unusable."
If it's a changeling then it was chemically altered. I see no detectable trace of original color anywhere in the left (blue) stamp. "Natural" changelings will still have some tiny specs somewhere where original pigment color can be seen. Chemical alteration changes the color uniformly. And just for clarity, Stamp Smarter isn't always right... but yes, purple and green can both yield blue changelings. (red pigment and yellow pigments are rendered neutral/transparent, and only the blue then shows through).
Comments
Ref:
Carlos Kullberg
Selos de Portugal - Álbum I (1853 / 1910) page 17
(Google translation)
"It is interesting to note that the seals (stamps) intended for telegraphic postage, were rendered useless with a five-point star perforation, after they took the mark of the day! Thus, stamps from issues from 1870 to 1893 were rendered unusable."
https://www.fep.up.pt/docentes/cpimenta/lazer/html/ebook/bfd003_p.pdf
And just for clarity, Stamp Smarter isn't always right... but yes, purple and green can both yield blue changelings. (red pigment and yellow pigments are rendered neutral/transparent, and only the blue then shows through).