Well they discontinued the 7.3 in 2003 but it does usually take more than a year to build a bus.. .So in the spirit of 0 being 1 for very high values of 0, it is possible that the chassis was built in late 2003 and it took until late 2004 to build the body.. possibly making it a 2005 model year bus on a 2003 model year chassis.
ive not seen a 7.3 on a 2005 bus.. im not sure it was legal to do so. as the EGR rules had kicked in such that after a certain date in 04 all had to have EGR which the 7.3 does not.. all of the navistar T-444E's for model year 04 seem to have had their engines / chassis built in the latter part pf 2003 as 04 models..
in fact i just ordered a later model computer for my 444E and the absolute latest serial numbers i could find were early december of 03. as 04 model year..