Here is a wing I picked up at a local shop about 1993. No serial number anywhere, not even under back plate. no model number. Tech at Washburn, Chicago thought it might be a prototype. rosewood fingerboard, gold hardware. Didn't realize until I had a local guitar tech look at it, that it was set up with 3 push/pull pots. two were for coil tap of each pickup and the third reversed the phasing of the neck pickup when the selector was set for both pickups. had to have been done at the custom shop when made as no one had tinkered with it till I had it looked at. other than that I know little about it. here are a couple pics.
Sounds like it may be a prototype. I've got a '92 wing re-issue that's not a neck-thru, that has a sticker for the serial number. The serial # says it's a '92, and there's no S at the beginning of the # (denoting the Samick factory). Washburn supposedly didn't make them in '92, so mine might be a limited run made prior to full production in '93 in Korea. Some Washburns are very hard to trace. Good luck!