First, is it a Couesnon Monopole model? Second, please give the serial number. Sadly, it appears that there are no Couesnon serial number/year lists in existance....after the factory fire. However, I've been able to determine the exact year of one of my Couesnon clarinets and an approximate year range for the other. I might be able to arrive at a ball park date. This will be helpful in determining a possible value.
One thing to know up-front is Couesnon clarinets are sadly undervalued. I got my Couesnon Monopole clarinets for surprisingly cheap prices. It's my impression that many clarinetists do not know about Couesnon clarinets and how excellent they are. It's kind of like how vintage Buescher saxophones were undervalued for many years.
I agree with your father. Vintage Couesnon clarinets are superb. Whenever I take one of my Couesnons to my repair tech for an adjustment (which is not often) he raves about the quality of its wood and craftsmanship. Truly, a modern Buffet cannot come close to the quality of wood in a vintage Couesnon.
Roger