Keith,
This very crufty old page (which dates back to the earliest days of this site, ie c1995) explains it in fair detail. You'll also find some details in the first few pages of the workshop manual.
The short explanation is this:
* The BODY ID is the manufacturing sequence for that BODYSTYLE, at that FACTORY
* The CHASSIS NUMBER is the assembly sequence for that MODEL, at that FACTORY
* The ENGINE NUMBER is the manufacture sequence of the block.
The last of these is the only one with a single sequence, as blocks were only cast in Melbourne.
The other two bear no real relationship to each other, since:
* wagon/van bodies were only pressed at Adelaide (until very late '59)
* only Adelaide pressed FE bodies until September '56, but the other plants assembled them. (My car is one of these - the ID plate is FE-225-5678 and the chassis 6-4760-M. So it's the 5,678th FE-225 manufactured in Adelaide, and the 3,760th FE assembled in Melbourne.)
* vehicles were assembled at Perth and Brisbane using bodies manufactured at other plants.
* the bloke who stamped the Sydney plates was blind and/or drunk most of the time
Hope that helps
RET