The original box would either be a Turner 5 speed (with or without overdrive top) T5-400 or an ENV 6 speed 542 SMOT.
IIRC, the 'OT' part of SMOT stood for overdrive top gear (though it's a long time ago . . . ).
If you have 6 (forward) gears then it sounds like you have an ENV box.
ENV became part of Eaton.
Manual available here:-
http://www.mark.clubaustin.co.uk/commercials.htm
and here:-
http://www.ploughbooksales.com.au/54.htm
Quote:
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010843 Bedford E.N.V. Transmission Type 542 SMOT Service Training Manual (photocopy), 12 pages, $15.00
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http://wakefield-files.freehostia.com/bedfordval.htm
Parts here:-
http://www.midlands.com.au/Gearbox.htm
(If you contact them they should be able to tell you where the identification plate is (or was) that will tell you what model of gearbox you have. Seems they also do reconditioned parts for Bedfords, so should know what you are talking about.)
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