Green’s Dictionary of Slang

bowler n.1

[bowl n. (1)]

(US drugs) an opium smoker.

[US]Goldin et al. DAUL 32/2: Bowler. An opium smoker. ‘Habit bowlers with plenty of dough don’t crack up like other junkeys.’.
[US]Anslinger & Tompkins Traffic In Narcotics 306: bowler. An opium smoker.