You are quite right in that these things are often a dilemma to solve.
Weapons smiths weren't necessarily fine writers; the are lots of their names wrongly spelled.
The barrel maker is often a person other than the lock maker; the similarity in both marks lettering would just be a coincidence.
Cerra doen'st sound spanish (Castillian); Cierra would sound better ... but Sierra would sound best.
Serrat is a Catalunian name and matches with the lock mark letters; who knows?
XVII century ? I would be deeply surprised