I saw this photo on another site, and it shows General Patton presumably in 1944-1945 in Germany. I would guess the gun is a captured "prize".
It appears to me to be a Jaeger sort of flintlock gun (I am not presuming it is a rifle). On the other hand, it appears to be large gun. Look at that fore end versus his hand. The ramrod tip looks mighty fat, like a shotgun, but that would be one heavy shotgun.
I just thought I would share this as a discussion starter. I am guessing Ed Wenger will have it meticulously replicated soon. :-D
I Googled that information on the sign but found nothing useful. That door construction says to me "Germany" or "Austria" or similar.
Wonder where this gun ended up?
God Bless, Marc