98 is the date the Fastex connector was manufactured. It doesn't really date the bayonet other than knowing that is wasn't manufactured prior to 1998. It's conceivable to find a second or third contract bayonet with a 92 dated Fastex.
Is there a difference between the 2nd and 3rd contract Lan-Cay M9 bayonets? Or is it literally impossible to distinguish between the two? Do you need pics?
Marv