The APM-6, launched in june 1981, two years after the APM-8, was in Japan named APM-6 because of its simpler 2-way arrangement - better for phase response and stricter sound dispersion.
Although normally simply named APM-6 or ESPRIT APM-6, it is almost always dubbed APM-6 MONITOR in Japan.

Akio Morita was very proud enough of the APM-6 to pose with it for a photograph in 1982.

The rounded shape of the enclosure was to avoid diffraction at the sides. Like the APM-8, the filter was SBMC encapsulated and made with the finest parts available as well as 99,99% LC-OFC copper.

While the APM-8 made it to the State of the Art award, the APM-6 MONITOR was awarded a Stereo Sound Best Product tag right after its official launch.

Production run is anybody's guess - around a 200 pairs ?

The top image comes from a 1982 japanese ad, the one to the right from a 1982 german catalog.
A japanese APM-dedicated website with plenty of images right here.