I have enclosed some images of original flintlock fowling pieces that have been converted to percussion using the drum and nipple all have the hooked breech.
I have also enclosed an image of a full stocked Fowler
Feltwad
A stand of conversion some proir to 1780
Notice the hooked breeches
A sxs flinter converted using the standard percussion breech plugs
A s/b fullstocked flinter with a hooked breech