Before attempting to beam to the mirror universe, Major Kira gets some phasers from Weapons Locker 47 in Ops. In the previous episode "Hard Time", Chief O'Brien almost kills himself with a phaser from Weapons Locker 47, which is in the cargo hold. As this is a parallel universe, and many things are the same, there is no reason why both stations should have the same numbering systems for everything. This is the classic "47" reference of Star Trek fame.