I think that it is a French 90 mm C.A. 39 S. which would make it a rarity as only 20 were completed before the French collapse. le Canon de 90 mm Schneider et ses dérivés Edit: The Germans completed those in production and, together with captured examples, had 36 in their inventory in January 1944.
Thank you, Roberto. if that is correct then I would feel better for not being able to identify it myself!