The serial numbers do not lie. They are what they are, but require some digging to determine who and where they came from.
The other Parts Division frame set had a BMX type fork. The serial numbers started with "WAK". I do not know which Japanese manufacturer used that prefix on their serial numbers.
I know this is an old tread, but I own both a WAK brazed frame and lugged 1983-84 frame of these bikes. Just to share my two cents after too long researching information a time ago. What I gathered was that the WAK frames were offered to dealers to promote their soon to be build Mountain Bike frame. I tracked the frame serials of a few and following the serial order there were at least 18 made (I imagine more). Richard Schwinn consulted with Marc Muller who built their 1982 Paramountain that displayed in the Chicago Interbike show. What they concluded was that these were Waterford made bikes. Serial number of the bike followed a January 1974 Schwinn serial but with a W placed in front. He told me that they may have used a stock BB. The frames were thinly brazed and may have had some interior support. These early frames lacked some cable braze-ons, they came with a straight tubular frame that was replaced under warranty by a Tange (pseudo biplane) fork, that is how I bought my frame. Attached is a NOS 1982 Brazed frame. Regarding the B,C-3,4-nnnnn frames what I found is that they were offered from 1983 to 1984 as shown in broshures and catalogs and came with a made in Japan decal. I had assume they had been made by Panasonic as they held business with road bike production.
The most important point about these bikes is that they somewhat discredit the idea that Shwinn wasn't trying to enter the Mountain Bike as early as Specialized or Univega and the Boutique MTB builders, but that it was experimenting, trying during difficult times for the company. The geometry was different that other bikes, with longer top tubes. Anyway that is all I have.




















