Eric Smith has scanned the Option/Module List (aka the Dick Best list)
and placed it online - search the archives and you'll find a pointer.
The M750 is on page 579 of the scanned document (the page is actually
numbered 577 in the top RH corner). As well as stating that it is used
in the 8 and that it is a "LINE I/O CONTROL", it also says that it is
equivalent to 2 W750s.
The W750 (p885) is listed as a Teletype Multiplexer.
Antonio
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Antonio Carlini arcarlini(a)iee.org