They were made by Laycock De Normville (sp?). The O/D ratio is 0.70 I think. I have one on my shop floor now. Bought it off of Ebay last year. I'm looking for the cut-out switch which goes on the carb linkage now. I found an NOS overdrive control switch through the Partsvoice site. I think MGs, Jags, and Volvo used versions of the same overdrive.