Theate like many Belgian makers, was a family business. Toussaint Theate started in Lieage in 1894. When he left the business, his sons took over as Theate Feres , Ernest and Adolphe 1907-1950. Eventually the company name was purchased by Raick Feres in 1950. It survived until 1965.
You can find the Belgian proof marks and date codes here:http://damascus-barrels.com/Belgian_All_Proofmarks.html
With out pictures it is impossible to judge the value of a gun. Especially as Theate Feres produced guns for such a long time. They offered standard field grades to some very nice guns.