Hi Fernando,
I too have little knowledge about firearms but I found this...I do not know if its helpful....but the stock is similar and is described as
A very Rare and unusual Georgian Half Sized New Land Pattern Tower Flintlock Musket,British Military Proof Marks,marked 24 bore , .577 cal. Barell 27 inch's , over all length 41.1/2ins. Regulation Lock with TOWER AND CROWN STAMP also maker stamp "JWR" ( J&W.Richards of London?)this would make it circa 1800.
I am not suggesting that its a twin...just your suspicion it is British, the crown mark and the JWR, I appreciate that the initials are believed to be
JPR but, I cannot see (on the photos how clear it is.......if the middle letter is severly 'wiped' the could the believed 'P' be a 'W')
The page below also has photographs.
http://www.northirishmilitaria.com/moreinfo.asp?id=128
Regards David