89 seems fine to me, that's a similar way Shimano etc date code thier publications.
If 8906 June's (Shimano use this style) then it is the same month as the patent* goes into the USA. But that's still going to be speculation.
*the patent (US) was applied for mid '89
A wheel for light vehicles and a disc member used therein. The wheel comprising a hub, a rim and a pair of disc members detachably coupled with the hub and the rim, wherein the wheel has the advantage of a spoke wheel that it is lightweight and has sufficient strength as well as the advantage of...
patents.google.com