This is an amazing weapon, Fernando and it has your name on it! Perhaps it was yours in a former life!
Not a gun person, but of course I must point out (if no one else previously has done already) the possibility of this being a naval piece. Brass and bronze blunderbuss were popular with sailors for obvious reasons. Bronze was resistant to the brine air of a ship which rapidly corroded iron weapons (It is why solid brass-hilted swords so unpopular with infantrymen due to sweaty hands and the possibility of dropping it was ignored by naval forces). There is a direct correlation with brass muskets and blunderbuss used in sea service.
The bayonet on yours does in fact appear to have been removed contemporary with its usage. The three main places you see these blunderbuss are at sea, used as defense as coach guns/to discourage robbery in shops and as defense on fort walls (these types usually mounted on a swivel). I'm not familiar of this type of bronze weapon ever being carried by foot troops. If naval, it is very possible that the bayonet was removed for practicality. On a ship, the blunderbuss main purpose was to discourage boarders clambering over the side of a ship, blowing a hole through a charging gang of mutineers or some such. Not much time or real purpose to having a bayonet. In any case, just my thoughts and I think you have an amazing piece for your collection!