Hi Michael,
The ID list i have is as follows -
Gear box identification
the identification marks can be found on an oval lug near the base of the box
G16A SD1 2000 (Could be 4 speed)
G17A SD1 2300/2600 5 speed
G19A SD1 3500 SE/VP 5 speed
G22A SD1 3500 Police spec 5 speed
G23A SD1 2600 Police spec 5 speed
G26A SD1 2400 Diesel 5 speed
G27A SD1 2000 Later model 5 speed
G28A SD1 Vitesse 5 speed
G30A Sherpa Range
Not sure about your having the higher 5th gear, ( 37 / 19 ) , maybe compare with a early box. If you have another box ( early one ) turn the input shaft when in 5th and see what the outshaft turns in relation.
These guys may be able to tell you more, what they dont know about these boxes is'nt worth knowing
http://www.ashcroft-transmissions.co.uk/index.php
Cheers John
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