G'day SD,
I've been looking at these clues in the IPC for a while. There is a couple of places where these little snippets appear. The thing that interests me is the jump from a 7 digit serial to an 8 digit appears in one short month (look at page 5). This occurs between September (9th month)and October (10th month). I can only assume that the 3rd and 4th digit if it exists is a month code. Either that or they sold close to 10000 machines in a month!!

What do you think? Ive tried to find a year code but don't have the raw data (I.e enough serial numbers) to work with.
I'd like others views on this. The Old Reds and the tractors seem to follow different numbering schemes.
Cheers,