What it is
The first MX-5050 line began with the silver-face upright. It also included the quarter-inch four-channel QX/QXD and half-inch eight-channel 8/8SD, whose electronics could sit apart from the transport. The two-channel upright could have an AC hysteresis-synchronous capstan driving a belted flywheel; DC-servo variants existed, and the QXD/8SD manual specifies a DC servo. Otari's US advertising sold the early line as the Mini Pro from 1974 (Billboard, 14 September 1974).
The separate-electronics machines documented in the same factory books — the 2SD₂ and ½QXD — are the Mark II-2 and II-4; see the Mark II.