First BK-1200 8 strings Bass was made 1979, produced by music shop "Key" with AK-1400 (Flying V shape guitar).
I believe these instruments were inspired by Greg Lake Scorpion 8 strings and AE-16 Flying V guitar.
AK-1400 Flying V guitar is well known, Ace Frehley played KISS live in Japan 1979.
BK-1200 8 strings Bass was Greco & Key store's ad with Tom Petersson in Japan Jam 2 Aug.1980.
Both instruments has nice Jacaranda veneer top and back with surrounded binding.
I didn't know the 5pin construction of BK bass, but I found the photo of the circuit.
It was simply passive 2 Pu construction, but switch phone jack mixed both PU when the plugged in, and each PU signal are send to 5 pin for stereo out. (No.2 and 4 pins & other pins are earthed)
After few years, BK-1200 were made with maple like top wood and mahogany body.
Eiji