I need to replace the chaine kit, but I saw that there are two kit for different production years.

first question: how I know if my 906 was made in 1989 or 1990?

second question: I can buy only the right one or I can swap from one to the other?

third question: where I can buy at the cheapest price the right kit?

many thanks

the size of the rear sprocket has changed, 38 old, 40 new (or vice versa )
accordingly changed the length of the chain

Yes, it was, especially if its not the cheapest one DID chain

if the chain is a zvm it´s a good price otherwise it´s the money not worth