The Z is just the prefix for the part. Most combine parts for the "late model STS" combines come with AH****** Numbers. Tractor's numbers usually start with RE****** you will see some Z****** numbers and even some R****** it is just the prefix attached to the beginning and no reference to the shape of the belt or part.
The Z prefix usually means the part is for combines or self propelled choppers manufactured in the German Zweibrücken harvester works. I once owned a 4435 combine so I'm very familiar with gold plated Z parts.
leonidas - Deere's prefixes are (or were) based on the general area or type machine they fit on. Generally, combine-specific parts begin with 'H' (or 'AH' if it's an assembly of some sort), cotton picker / stripper parts begin with 'N' ('AN' for assemblies), and tractor parts with 'R' ('AR' for assemblies). Sometimes you'll find a combine belt with a "n" prefixed number on it. This is a belt that was originally used on a picker, but dimensionally is the same thing needed on the combine. Pretty much hard and fast rules through about the '80's. But now there's so many different models, the part numbers are all over the place. Deere also went with the 6 numeric digits a while back to allow for more part numbers w/in the different categories ('N', 'R', 'H', etc). The numeric part of you belt part number has no correlation at all to the belt's size. These are only my opinions, based on working in a Deere parts department from '86 to '96.
I couldn't have said it better. The only thing I will add is that the numbers obviously increase over the years as they add more parts. You can tell roughly when a part was originally put into production by the number. When the 9600 was introduced in '89 there were many new part numbers around 125000. eg AH128449. At the introduction of the STS in 2000 most numbers were around 170000. They hit 200000 in about 2003. This is for NA harvester parts but I believe the other plants follow the same rule. Oh, "R" and "AR" is Waterloo and "L" and "AL" is Manheim BTW.