Slightly later, smaller version of the grand 5K50 which was added to the first Laboratory Reference Series in 1977/78.
Realtime Processed DC Amps (DC rec and play signal paths), Sendust Alloy Heads, reel-base motor physically distant from the capstan motor, 38-pulses FG servo capstan motor isolated with 3-fold magnetic shielding and anti-vibration rubber, full-IC feather-touch transport pads and Plasma level meters with fast 0,7s decay (no "plasma" here : just a regular FL tube).
Four guide rollers for constant back-tension, wide pad and new holding mechanism for constant pressure and stable frequency response, SKEW adjustment to obtain best azimuth and tilt heads position (with the XM tapes), high-precision roller with reflection plate to read counter real time (5K50 and K-12).
Dolby B NR, Metal tape compatibility (Type IV - EX), independent bias and eq settings, MIC mixing circuit, memory counter rewind, optional wired remote (with added auto-rewind and auto-play modes), timer rec/play switch and Rec Mute function.
The back hides a tiny switch to use the digital counter as "auto" or as "time counter"...
Otherwise... extra good looks (available here and there with an x-rare wood enclosure) and sturdy build-quality.
The K-15 topper added a third head for serious recording and tape/source monitoring but the K-12 was already very expensive for a mere two-head recorder : 995$ in 1980 !
Mechansism-wise, I still don't know if this was already Alpine-inside or not or, as for the 5K50, this was a home-made item in Luxman's newly-bought recorder factory...
If the K-10 was made by Copal Koki, the K-12 bears a Lux-A T-tag... and "A" sounds very much like Alpine.