035400 was the cone number on the later 515 416 604 units. It appears as the cone number for many Altec units that are of similar ilk, i.e. 416/515/604 in the 1999 EVI/Altec parts cross reference. AFAIK it is the updated 21184-x cone, used in the 416/515/604 series.
How different it sounds, I don't know. Bowtie has pointed out that GPA add the GPA prefix and use many of the Altec part numbers from the time when they purchased the tooling, hence GPA 35400 is probably the same as an 35400. It may be possible that early GPA cones did not have the GPA prefix, just to muddy things.