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.
It's weird though as the FW800 is the most 'advanced' version of the powermac G4 (the MDD 2003 was actually the same as the 2002 version). The chip also has the 3-phase voltage regulator just like the 1.42GHZ model which is definately a 7455B.
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