Overdrive

[30psi]

A 5 speed box is a 4 speed box with O/D, its just a different marketing method to describe them

[Ronin]

The Scimitar box had a button on it you had to press to engage O/D so you would have 1st, 1 1/2 (O/D), 2nd, 2 1/2 (O/D) and so on up the box. If you've seen Mark Evans An MG is Born on Discovery he fitted a similar box to the MG B he built and he actually went to the place where the boxes are made/reconditioned.

[30psi]

That sounds like it works more like what some would refer to a 4+3, much like one of the corvette box's which is a 4+3, in the sense that you can overdrive to 3 of the gears and in essence becomes a 7 speed.