Are there any references to EM-33 pistols outside of ST:Enterprise?
Is there an exact date as to when the EM-33s were completely phased out (even on freighters)?
The EM33 Plasma Pistol is described as the "standard issue EM weapon used by Earth's Starfleet officers and the Earth Cargo Service personnel during the mid-22nd century" and is used extensively in the episodes "Broken Bow" (first seen), "Sleeping Dogs", "Fortunate Son", "Shadows of P'Jem" and "Canamar" (where it was mistakenly identified as an alien weapon).
The designation "EM-33" comes from the episode "Fight or Flight" where Ensign Hoshi Sato says she's been trained on EM sidearms; Archer then tells her the Phase Pistol handles "pretty much like an EM-33."
Certainly, by the time of Enterprise:TOS they've moved onto the Type-2 Phaser.
There are some nice prop photos here. Apparently the pistol itself is a heavily modified Desert Eagle moulded out of plastic.