Probably not a PN change unless its part of a SB or AD, most probably a mod status change.
Both part numbers serve the same purposes and meet the same TCAS spec but the manufacturers and system are different the first pn is more of an all in one...
Could you confirm whether the part number of the TCAS computer is altered as a result of the upgrade from software version 7.0 to version 7.1?
Could you provide clarification regarding the technical and configuration differences between TCAS computer part number 7517900-10020 and part number 066-50000-2220?
Most lower cargo compartments will have a heating system.
On the 767, hot engine bleed air is fed into a long tube running the length of the cargo compartment floor. The tube has holes to disperse the heated air.