Mystery Gun
Mildman,
The hammer, curved base on the lockplate and triggerguard with the shallow rear all look German or Bavarian but I cannot find any matches for the right angled plate opposite the lock (that plate is usually a graceful curve on German and Bavarian muskets). The barrelbands are the type usually associated with pre-war flintlocks and the bolster sure looks like a conversion.
I'll keep looking for a match for the plate, but it looks to me to be an arsenal conversion of an German/Bavarian pre-war flintlock.
Just a humble opinion,
TomH