Andy
Here in Australia it is 406 for the PLB's. As you say they also transmit on 121.5 to assist rescue aircraft with the last part of homing during the rescue phase.
The up to 2 hours before a fix on the 406 signal is for the non GPS version - they need a low earth orbit satellite to get a doppler result to establish position.
With the GPS version of the 406PLB (or EPIRB in marine case) the GPS position is known virtually instantaneously as the Geostationary satellite picks up the initial 406 signal and in that signal the GPS position is also available.
John