dave is right you just override the switch to give overdrive in all gears , it is worked on a jag or or other bl car by a switch on the top of the gear knob so to use in first and second as an 8 speed you have to be very fast on the switch
easyer to use as a 6 speed with a high or normal 1st and 2nd then 3rd, 3rd high, 4th and 4th overdrive i think the one on my old jag box would have made it impractical to use as anything but a 4=overdrive as the ratio for 3rd high and 4th would be the same near enough as would make no difference
i dont have the ratio for the triumph one or the rover one but think they were the same