I think there's another possible clue and that is the reference to the 1872 Exposition de Musique. While that doesn't specifically date the instrument, it suggests that the model was either a prize winner at that event, was introduced then, or someone possibly received the actual instrument as a prize at that event. I don't believe there's any question that this is a very old instrument. Whether it has historical significance is probably what you're searching for. Not much but hopefully might lead you to something.
Thanks everyone so far:
French-Courtois imported to Chapells
The 1872 is still vague
I’ve found one other from similar era but no exact match
The charity who owns it is going to put it on eBay