Borrowed from Wikipedia:
The vast majority of overdrives in European cars were invented and developed by a man called de Normanville and manufactured by an English company called Laycock Engineering (later GKN Laycock), at its Little London Road site in Sheffield. The system was devised by Captain Edgar J de Normanville (1882–1968),[2] and made by Laycock through a chance meeting with a Laycock Products Engineer. De Normanville overdrives were found in vehicles manufactured by Standard-Triumph, who were first, followed by Ford, BMC and British Leyland, Jaguar, Rootes Group and Volvo to name only a few. Another British company, the former aircraft builder Fairey, built a successful all-mechanical unit for the Land Rover, which is still in production in America today.
The first production vehicle to feature the Laycock system was the 1948 Standard Vanguard Saloon. The first unit to be created was the A-type overdrive, which was fitted to many sports cars during the 1950s, and into the late 1960s. Several famous marques used A-type overdrives, including Jaguar, Aston Martin, Ferrari, Austin-Healey, Jensen, Bristol, AC, Armstrong Siddeley and Triumph's TR sports car range, from the TR2 through to the end of the 1972 model year of the TR6.
In 1959, the Laycock Engineering Company introduced the D-type overdrive, which was fitted to a variety of motor cars including Volvo 120 and 1800s, Sunbeam Alpines and Rapiers, Triumph Spitfires, and also 1962-1967 MGBs (those with 3-synchro transmissions).
From 1967 the LH-type overdrive was introduced, and this featured in a variety of models, including 1968–1980 MGBs, the MGC, the Ford Zephyr, early Reliant Scimitars, TVRs, and Gilberns.
The J-type overdrive was introduced in the late 1960s, and was adapted to fit Volvo, Triumph, Vauxhall/Opel, American Motors and Chrysler motorcars, and Ford Transit vans.
The P-type overdrive marked the last updates and included both a Gear Vendors U.S. version and a Volvo version. The Volvo version kept the same package size as the J-type but with the updated 18 element freewheel and stronger splines through the planet carrier. The Gear Vendors U.S. version uses a larger 1.375 outer diameter output shaft for higher capacity and a longer rear case.
Over a period of 40 years, Laycock Engineering manufactured over three and a half million overdrive Units, and over one million of these were fitted to Volvo motorcars.
In 2008 the U.S. company Gear Vendors, Inc.[3] of El Cajon, California purchased all the overdrive assets of GKN to continue production of the U.S. version and all spares for J and P types worldwide.
The system features an oil pressure operated device attached to the back of the standard gearbox operating on the gearbox output shaft. Through a system of oil pressure, solenoids and pistons, the overdrive would drop the revs on whatever gears it was used on by 22% (.778). For instance, the overdrive system applied to a Triumph TR5 operates on 2nd, 3rd and top gear. When engaged, the overdrive would drop the revs from 3000 by 666 RPM, or from 3500 the drop would be 777 RPM to 2723 net. The advantages this reduced rpm had on fuel consumption was most often quite near 22% decrease during highway driving.
I don't have any idea whether or not Gear Vendors has anything for the type D / Alpine overdrive unit, but they are the successor to Laycock and GKN. I do know that Gear Vendors overdrive units are used in some enormously powerful rear drive cars.
