Hi 'Nando,
Another nice piece for your cannon collection!
I would date this littly cannon guy to the second half of the 15th century, the large touch hole addmittedly being the only criterion.
You know my opinion, amigo: please do not change anything about the surface as you say it's stable and not losing material.
I wouldn't even apply olive oil because this would blacken all the charming little places of brown and yellow discoloring that denote its great age as well as a certain period of time spent underground.
Originally this littly guy, too, may have been part of some multibarrel cannon construction of which I posted several samples before.
Congratulations and best wishes,
Miguel