[Linked Image from photos.smugmug.com]

Top to bottom:
Probably 'PF' and likely the Twist barrel maker who I can't ID
Possibly the inspector's mark. If crowned, prior to 1877.
FD is likely Ferdinand Drissen active 1874-1900
On L the Provisional Proof mark and bore 22.8mm at the time of 1st obligatory proof. On R the Belgian Proof House mark.
No way to date the gun further.