The Portuguese artillery employed just about any brand of guns: Krupp, Schneider, Vickers.
Luckily we have the Vickers online archives and it shows some Portuguese guns:
A 3 inch aa gun
a 4,7 inch naval gun
And check out the mount of this fixed 3,7 inch, it is smiliar to the one on Alto de Sao Joao
So is it a Vickers 3,7 inch?