knife cousin?
When I looked at that knife, some thing was familiar about it.
Sorry, I am not able to photograph my TEKNA knife, but the blade is similar. It is full tang, but without the scales and guard.
I believe it to be a divers' knife; stainless.
It too has a unique sheath. The handle is lightened with five holes that are 1/2 inch diameter, and lines of ribs on the edges for a grip. The heavy molded plastic sheath has a "window" I suppose for water drainage, and where the knife enters is a flap on a hinge. Spring held.The inside of the flap has a circular projection that engages the lowest circular hole in the handle and secures the knife in place.
Very similar to a handgun holster snap retention. You just place your thumb inside this flap and press it to release the knife on draw. The dimentions you've give are similar too.