They are actually called 'Marjacq'
They are being made by the Arno company in Theirs, France. They have revived the style, apparently used in the Savoy region previously.
I can't find a web site here in France.. From this French web site selling them I can glean the following:
HM Diffusion - MARJACQ OLIVIER, BUIS OU EBÈNE
This page has a few images including how the lock works and this nice image of the three models they make:
Key Details:
Overall length: 20cm. The handle is 11.5cm/ 4.5" opened overall length.
Blade: The blade is made from stainless steel and blade length is 8.5cm/ 3.3".
X46Cr13, hardness ca. 57 RC
Security locking ring: (open and shut) is also made from stainless steel.
Handle: Boxwood, Olive wood, Ebony
A quick translation:
The Marjacq knives, manufactured in Thiers (famous knife making town in France), are perfectly suited for leisure use or for the workshop.
Stainless Blade and Locking Ring
The blade and the blade locking mechanism/ ferrule ( it is possible to lock the blade in the open or closed position) are made out of stainless steel. Their production is thoroughly followed in order to offer the highest level of quality control.
Olive wood Boxwood or Ebony Handles
The "Marjacq" are offered with 3 different handle materials: Olive, Boxwood and Ebony. A small notch is formed on each handle to ease the opening of the blade. Each handle is finely sanded then buffed by hand (polishing with a flannel) for a very soft touch and a superb development of wood grain.
Assembled By Hand
The final assembly of the various parts of the knives is carried out entirely manually. It is the assurance of a top quality finish.
The Marjacq knives are available separately, in batch of 3 knives (one of each handle type) or in batch of 6 parts (assorted handles).
French site cost is 14 for olive and boxwood. 18 for the ebony
DFT has more info and steel details:
The revival of a legend: The Marjacq Knife
For the rural inhabitants of the Savoy region of France, the Marjacq knife has been the utility knife of choice for centuries. It is foldable, yet remarkably solid and perfectly designed to fit the hand comfortably, making it ideal for performing everyday cutting tasks as well as preparing and eating food. The Arno Company, one of Frances leading makers of traditional knives, has recently resumed making these classic cutting tools. The elegantly curved handle is not only comfortable to use, it also completely hides the blade when folded. A simple yet reliable locking mechanism securely locks the blade both in the folded and unfolded position. Both the locking ring and blade are made of hardened stainless steel (X46Cr13, hardness ca. 57 RC).
I see that Axminster say they offer a Beech wood £7.27 and Dick Fine Tools offers a horn version at 45.82 so assume there are more handles materials. DFT states they weigh 60g
I guess the ebony model would be less liable to swelling?