This was post- Music Link series and a strictly Marantz JP development.
The SL-1 is based on two resistance arrays very probably built by Tokyo Ko-On Denpa, a specialist who provided items to Pioneer for its Exclusive series and the A-09, Micro for its CL-M2DC and Luxman as well for the C-10 / C-10II and lower variants.
The contacts here have been silver-plated ; other contacts such as that of the input selector are gold-plated and all wiring is done with OFC copper. The case is like that of an SC-22 : 8mm front, 3mm top, diecast aluminium chassis and "special coating" of the side panels for vibration damping. Four inputs available plus a fifth named "off" but linked to the "direct" terminals ; three outputs available : 1, 2 and "direct", the latter turning the SL-1 a transparent pass-through (when used with the "direct" input).
Available in champagne color later on but neither sold in significant numbers (to say the least).
A real SL-1B here ; a real SL-1N here.