The Enfield on the lock shows that it is a replication of a gun made at the Royal Small Arms Factory at Enfield Lock, North London. There should also be a circular stamp on the right-hand side of the butt with the word 'ENFIELD'. The originals, and any replicas, therefore replicate a firearm that shot/shoots the .577cal Minié bullet [called the .58 for the US market], and does not need a patch, hence the lack of a patchbox. The Musketoon is a very short - carbine length - firearm, with a barrel length of not more than about 24 inches. A two-band, or 'short' rifle, has a barrel of around 32 inches. That will not have a patchbox either, for the same reason. Navy Arms replicas are made in Italy, and therefore will have Italian proof marks somewhere on the breech end of the barrel. Yours SHOULD have a set of letters in a square cartouche. Tell us what they are and we'll tell you how old it is, but it's unlikely to be more than about eighteen for the reasons detailed in the second to last line below.
The DG in the circle is the maker - Armi San Paolo S.r.l.. Established in 1970, it uses the last names of the original founders Grassi, Doninelli, and Gazzola as a symbol, DGG, usually contained within a circle. Beginning December 31, 2001, Armi San Paolo officially became Euroarms Italia S.r.l. The same logo is used on all Euroarms models.