Hi Yannis,
According to Forton's "Navajas Antiguas", the standard reference book for collectors, that is a French navaja from Chatellerault, and dating from the middle of the 19tch century. In my opinion it could be a bit later, but is almost certain to have been made before 1900.
Re the lock: From your description,it it is a""teat lock"; The teat being that metal protrusin that engages with the back spring. It was customary to fit the back-spring with a ring that could be pulled with the finger and disengage the lock more easily. I wouldn't be surprised if yours was originally fitted with one and subsequently lost.
Contrary to the wishful thinking of incurable romantics, most utility navaja, those intended for actual use, rarely had blades in excess of 9", most being in the range of 5"-8. Reason being that once a blade went over 9"or so, the assembly became very weak and the folder broke easily, usually at the hinge. Of course handles could be made robust, but then they became too heavy and cumbersome. Once one sees decorative etching on the blade, it is near certain that the knife was intended to serve as a display piece or souvenir. Most large navajas were of this ilk.
That knife, assuming that it is in as fine condition as what the photos suggest, is worth a fair bit of money. If you want to get an idea as to how much similar knives retail for, I suggest that you visit this website from time to time:
http://www.knivescollection.com/cata..._antichi_e.asp
Cheers
Chris