Sabatier Trumpet?

It's hard to say. Sabatier used to describe a high quality knife made by a select few manufacturers. Later on the logo was put on a bunch of mass-produced knives that were garbage. So, it's hard to tell if it's good or not; you'd probably need to take it to someone that has experience in those.

Tuumpet or trompette? Does it have a diamond mark?

clean it, sharpen it, and enjoy.

Your knife is a Sabatier Trumpet Export.  The combination of the brand, handle style -- a block of ebony on a rat tail tang; and the rest of the design -- cylindrical bolster and classic, full-finger guard type bolster -- dates the knife from the early fifties to the mid sixties.   Pretty much all "Sabatiers" of the type and period were made of more or less the same steel, with very similar profiles and have very similar characteristics. 

The style has come to be known as "Nogent" in the U.S., but the term is problematic.

Your knife is not of any collector or historical value, and must be used to be enjoyed.

The knife should be easy to sharpen, hold an edge well, easy to maintain on an appropriate steel (used appropriately).  The 10" size is long enough for nearly any slicing job, and short enough for almost all trimming.  The handle style is comfortable for nearly all hands.  Fantastic knife, really.

Based on your pictures, it doesn't appear the knife has been used very much nor sharpened very often -- so the edge geometry should be in good shape.

You'll want to scrub the stains off with a "Scotch-Brite" pad.  As said, BKF can be helpful.  However, BKF contains oxalic acid which will cause your knives to stain again almost immediately after using it.  If you do, make sure to neutralize it with baking soda, immediately.  You may use baking soda without BKF to remove the stains.  It will take more time and elbow grease, but... there you go.

Old carbon Sabs are very easy to sharpen.  You don't need water stones or anything special.  A good set of India and Arkansas stones will do the knife proud.  In any case, use quality sharpeners.  Don't use cheap "diamond" steels, or carbide sharpeners, or "tackle box" sharpeners -- they'll quickly destroy the knife. 

BDL

They are nice knives,much like yourself I found a rusted one,cleaned and sharpened it is a biggie(it is 17") a little to big to be practical,and saw the same on  e-bay for$ 130  ,so thats nice,and nice quality,it is worth cleaning and using,or sell it...yer friend caveman