What it is
The MX-5050B is the second generation of the upright line — the machine that followed the silver-face Mini Pro. Otari was advertising the "all-new MX-5050-B" by August 1978 (Recording Engineer/Producer, August 1978), a year before its MX-5050-B brochure took effect in September 1979 (brochure scan). Its life in the trade press is on the History page. It is not one transport but a split line: the 2H, 2L and 4L keep the first-generation AC hysteresis-synchronous capstan driving a belted flywheel, while the D-suffix models (2HD, 2LD, 4LD…) get DC-servo direct drive. The generation table on the History page reads it across: a plain-B 2H/2L/4L transport is closer to a Mini Pro's than to a BII's.
Specifications
The B specifications page compares Otari's September 1979 two-channel sales figures with the OS2-004 maintenance manual's alignment and performance tables for the wider series. The brochure describes a DC-servo example; the maintenance manual also covers the AC-synchronous 2H, 2L and 4L.
Have one, or paper for it?
The instruction manual OS1-011 is now held as a Library of Congress scan, with a paper original awaiting receipt. An original copy of the 1979 brochure, or a rear-plate photo with the suffix legible would each turn this page from an outline into a reference. See how to contribute.
Sources
MX-5050B factory documents
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MX-5050-B brochure, September 1979 — Otari's sales sheet, scanned and transcribed here.
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Maintenance manual OS2-004 — the synchronous and DC-servo variants, with their alignment tables.
First generation and wider family
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History: where the Mark I fits — the generation table and the capstan story.
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The wider family — where the B sits beside the MARK line.