Its the 7555A I bet. The A signifies it is an older chip than the ones in the guides you used which were fine to 1.5GHz. The longer a production run continues, the more the process gets refined and the higher the yield will be. Higher yield = more chips which will run at higher speeds. The fact that they changed the revision implies that they made a larger change to the process of manufacture than just the usual day-to-day tweaking.