Hal, the 16.8 refers to bore size in millimeters, which is 0.661 inches, or 16 gauge. As to the stampings, it could be "NL" or, reversed, "TN". Either way, with the "H" mark, I've not been able to trace these to any maker or craftsman with the resources I have. Perhaps someone who recognizes these from their own research will chime in, one can only hope! Your pictures were very clear. Unfortunately the letter stampings were not.

To your earlier questions. If you are referring to tiny protrusions at the top of the breech face that are lined up with the pin holes on the barrels, one frequent contributor to this thread named them "keepers", as they help line up the pins and keep them perfectly upright. They vary in shape and prominence, from barely there, to quite prominent and fitting into a squared recess in the barrel rim on some guns. They might act as a gas seal or gas deflector (by reducing the size of the pin hole), or simply to 'round' the pin hole on the barrel, which would otherwise have a sideways 'D' shape. Here is a picture where you can see them protruding from the top of the breech face:

[Linked Image from i.imgur.com]

Not all pinfires have them. In my mind filing a perfectly flat breech face is hard enough, but leaving two small protrusions? Too much work!

Yes, the fore-end does look like a casting, as do mine.

As to the accepted term for this kind of action, I suppose it is most commonly referred to as a "Lefaucheux" action, as per its inventor. However this is far from precise, as Casimir Lefaucheux designed quite a few actions, and his pinfire creation underwent an evolution by his own hands, as AaronN has shown us, let alone all the minor variations introduced by other gunmakers of the period.

From the Liège proofmarks, your gun was made after 1852, but that's about as much as can be told.

Light pitting in the bores is good news. The insides of pinfire barrels are often quite horrific from neglect.