wicket here, a small gate separating the kitchen from the dining room of the diner.
kosher convent To most people, a convent is associated with Catholicism; here, Max jokes that Al, probably Jewish, would have to be in a "kosher" convent; kosher is Yiddish for food that is ritually clean, according to dietary laws.
muzzle of a sawed-off shotgun the firing end of the gun.
the car-tracks The reference is to electric streetcar tracks.
