The Daw patent is no. 1594 of 1862 which is a set of improvements to the Schneider patent no 1487 of 1861 to which Daw had purchased the rights.
These patents are described quite fully by Crudgington & Baker in The British Shotgun Volume 1, p.100-103.
Sadly there are no schematic drawings of the patent.
I was fortunate to have one of these Daw shotguns, no 766, a few years back and could examine the way the whole thing worked and was very impressed. I don't think the lever is protected by the patent, Purdey may have latched on to the monica simply because Daw was a profligate gunmaker, used a lot of underlevers and became associated with the style.
I too use patents to date guns for import/export documentation but one has to be very careful. Unless the patent is credited on the mechanism by stamps, engraving or whatever, they can not be assumed to have been used within the protection of the patent (max 14 years) although they may give an 'earliest' date.
But yes, it does sometimes give an indication of date.
As regards the NFB mark, try the legislation date as all changes to proof rules are by act of parliment but again be careful. The proof houses are suspected for using out of date stamps for a period after a change became official and omitting new stamps for a while.
An example is the date stamps introduced by both London and Birmingham. I have seen several guns from well into the period of date stamping with none!
