It looks to me like it might have started life as a flintlock & been converted to percussion with a drum & nipple conversion, As Enfield 2 band says its either blown of or broken off. What is the brass colour? has someone brazed the breech? whats the bore like?
Hollis was a good maker but in that condition it has minimal value, similar complete relic guns can be had for a hundred quid so a barrel on its own?