After the foundation of its USA subsidiary in 1975, Lux was pushing all directions at the same time : Luxman, L&G, Luxkit, high- mid- and low-end for all. This is when the crazy multiplication of lineups and products really started in full.
The 5K50 is part of the later Laboratory Reference Series and was Luxman's first foray into recording machines, seemingly made in a recorder factory it had bought for the purpose in 1977 (perhaps Dokorder, soon to be acquired by Murata in '82).
Until that point receives factual data, I don't know who built the early Lux recorders but it is very likely to be Alpine... T-tag for the 5K50 is "LUX-A" :)
The drive is a bigga assembly mostly made from diecast aluminium with a removable head-stack assembly, just like on a Technics RS-1500U.
The heads are made of Sendust for playback and Ferrite for recording, with 1µ and 4µ gaps, respectively. Azimuth can be adjusted, of course, like on a Nak : one screw, two LEDs, both light up, you're set.
Inheritance from the "pro" world, a review/cue lever was added under the stop pad for quick search in "pro" style.
Circuit-wise, Luxman carried its Realtime Processed DC amp in both the rec and play amplifiers and variable bias control with BRBS : Bridge Recording by Bias Current and Signal Current.
The audio signal and bias currents are overlapped at the bridge circuit without coils or capacitors in the NFb loop of the recording amp : the peaking coil and trap circuit are eliminated for less transient distortion and phase shifts.
The built-in oscillator pushes 1Khz or 6Khz signals, originally written on the front plate (1Khz left, 6Khz right), later on changed to Bias & Dolby cal. (left) and azimuth (right). Bias is continuously variable for EX tapes.
The digital counter only reads numbers, not time, but if XM tapes were used, real-time display was possible, in minutes and seconds.
The chassis holds a bigga toroidal trafo (really big), solid steel plates, one large r/p board with many regulations and separate power-supplies.
Grand looks from Luxman's heyday, too : finely chiseled knob design only equalled by Sony (and vice versa), assymetric and perfectly balanced layout and a 12-dot "Plasma Level Meter" which is a normal FL tube but Plasma sounds so much better :)
Searching for accurate Luxman release dates is often difficult as more often than not 1970s export catalogs aren't dated and original (japanese) single-component catalogs are more like b/w sheets with little info (or date).
According to official "bibles", the K-12 came first, then the 5K50. But the K-12 has Type IV compatibility, the 5K50 hasn't.
New Product sections in japanese magazines announce the 5K50 as brand new in early 1979 and that it is an "upped" version of the K-12. But a japanese Luxman "1978 new product" sheet shows well the 5K50 - contradictory, isn't it ?
Since Type IV became standadized only in 1979 and the 5K50 can be seen on the cover of the december 1978 MJ magazine (only the cover : nothing inside about it), the 5K50 should be first in october/november/whenever 1978, then the K-12, then the 5K50M in 1980.
All are very rare at any rate.