Bus Depot has a nice (though admittedly partial?) explanation of this:

http://busdepot.com/links.jsp#NUMBERING

Will in Alaska.

On Sun, 2009-11-22 at 19:23 -0500, Jonce Fancher wrote:
> Hi all
> Not sure if some one can send me to somewhere to find out what the
> Model Numbers for a part number are.
>
> Like a VW part number 025 109 612A. The 025 is Vanagon Exhaust Valve.
> Now if it is a 043 109 612A What VW is This Exhaust Valve for?
> Now if it is a 1H0 109 612A What VW is This Exhaust Valve for?
>
> Is there a Prefix showing ALL Prefixes?
> I've not been able to find one.
> I have parts with 022,025,026,027 and a ton of others.
> Any help would be great. Or just a link to all.
>
> Thanks. You can email me direct if you don't want to post this to the list.
> Thanks
>