First entry is completely correct.
But to add some interesting info... The term 'whistle' is used a multitude of times throughout HBO's, 'The Wire'. Although it is never distinguished or addressed directly, it becomes apparent that the term 'whistle' really refers to ONLY a pistol, a semi-matic handgun.
David Simon's bestseller book and basically the father/catalyst of The Wire is 'Homicide'. Again the term 'whistle' is NOT explicitly defined) (Wether its only a pistol, or maybe a type of Mac/Tec or a shotgun) However the book does make it clear that 'whistle' is a term used in Baltimore. This isn't explained out right, but through stories, recorded conversations with citizens and the times when slang was recorded throughout the book, it becomes abundantly clear that the use of the term is authentic, meaning 'whistle' is actively used there.
>> "Yo we movin' out now, make sure you grab yo whistle!"
>>"Yo man, what if someone jumps out on us, rolls up on our corner?" "Nah its straight man. Check it, Got dat whistle layin over there..." (Points at a pistol sitting on top of a tire on a car, semi hidden between the wheel well and the tire, but close to the corner/stash)
>> "Nigga, either sound off wit dat whistle or tuck it, you be obvious as shit right now!"