Don't have a definitive last year made number but I do have this info on when the changes were made on 643's and how to decipher JD serial numbers on combines and heads. Not sure if this still carries through on latest up to date equipment or not.

643 Serial Numbers

100001-150000 1974
150001-200000 1975
200001-250000 1976
250001-300000 1977
300001-350000 1978 start of the pedestal on left side of the feeder house moved over for reverser
350001-400000 1979 start of low tin, chains still run dry
450001-500000 1980
500001-550000 1981 start of drive chains running in oil

This list has been especially helpful when looking at some used heads and seeming them advertised as newer than they actually are. (bidding leverage)

Both of these lists were shared here previously so cannot comment on absolute veracity of info.

John Deere made heads and combine serial numbers fairly easy to translate. The breakdown goes like this. All serial numbers you are interested in, end with 6 numbers. The first three will identify year model. I will give you some examples and they break like this. 1989 starts with 630xxx. 1990-635xxx 1991-640xxx and so on. 1998-675xxx, 1999-680xxx, 2000-685xxx, 2001-690xxx, and so on. 5000 numbers available for each year production. Hope this helps.

The breakdown goes as this. All serial numbers you are interested in, end with 6 numbers. The first three will identify year model.

1989 starts with 630xxx.

1990 starts with 635xxx

1991 starts with 640xxx

1992 starts with 645xxx

1993 starts with 650xxx

1994 starts with 655xxx

1995 starts with 660xxx

1996 starts with 665xxx

1997 starts with 670xxx

1998 starts with 675xxx

1999 starts with 680xxx

2000 starts with 685xxx

2001 starts with 690xxx

And so on.

5000 numbers available for each year's production.

I'm pretty sure 90 series came out in '95 so '94 last year for the 40 series.  Reason I think this is right is a new one was bought with a new '95 model 9400 for the '95 crop year.  

That's fairly indisputable evidence right there.