AEC Ltd (or the Associated Equipment Company as it was known prior to October 1948), grew out of a reorganisation, that occurred a few years before WW1, of the London General Omnibus Company or LGOC that could date its origins to the mid 19th century.

Ostensibly a chassis and engine/running gear manufacturer, AEC became one of the most successful and prominent suppliers of running-units in the truck and bus industry, supplying London Transport and many other companies (and countries) besides. From 1933 it was based at Windmill Lane, Southall, in west London.

In 1948 it formed Associated Commercial Vehicles Ltd. (ACV), by the amalgamation of itself with the Maudslay & Crossley concerns, although these were left largely independent, whilst AEC continued to be a major force in the truck industry.  Shortly thereafter though (during 1949), ACV was to take over the coachbuilders Park Royal Vehicles (PRV) creating the "happy marriage" of running-unit manufacture and coachbuilding expertise (see the note below). This combination was so formidable in the bus and coach industry that it was unstoppable in winning major contracts both throughout the country & worldwide and went on to provide, almost exclusively, London's buses (including the Routemaster) for the next thirty-plus years.  

The involvement of Leyland from 1962 was not of itself significant, but after Leyland's later nationalisation stemming from its disastrous attempts to return Britain's car industry to profitability, it became so.  It should be noted though, that the ACV Group and Leyland had been bitter rivals, especially in the early days of the Routemaster and in the early 60's when AEC had strongly opposed London Transport's suggestion that Leyland would be commissioned to supply running units for the vehicle.  AEC blocked the potential deal between LT & Leyland by forcing LT to adhere to its long held purchasing agreements.  This rivalry became a significant issue creating much antagonism between the former ACV companies and Leyland.  Leyland had promoted its Atlantean chassis as the group's main product in the commercial passenger vehicle market over anything that AEC was producing, leaving the latter supporting its diminishing market for PSV chassis and Routemaster running units.  Leyland even introduced its Ergomatic cab across all its truck ranges further reducing AEC's own brand value.  As Leyland began providing more of the chassis production, after much right-sizing (the euphemism for plant closures and job losses), it became evident that the AEC facility at Southall was surplus to needs and thus it was closed in 1979.  And AEC's sister company PRV followed AEC's demise the next year after production of the B15 (Titan), that was designed and built at PRV, was moved to Leyland's factory at Workington, Cumbria.

An Autocar Services Ltd., of Maidstone/Tunbridge Wells single decker circa mid 20's maybe based on an AEC 413 chassis, one of KM 8930/1 or KM 9107/8.  The livery was a distinctive purple & ivory.

The original Autocar firm began operations in 1909 but in 1928 became part of London General Omnibus Co. Ltd..  However, upon the formation of London Transport, as most of the operations were outside London Transport's area, in 1933 it became wholly owned by Maidstone & District Motor Services and all Autocar vehicles were transferred to that company, excepting the London routes that became part of the Green Line network.  Maidstone & District wound up Autocar Services in 1935. (A new Autocar firm set up in 1997 and operating in the same area, in respect of its heritage name, retains the livery used by the original company.)

Note: THE RELATIONSHIP BETWEEN ACV, AEC & PRV: A common fallacy is that PRV was a subsidiary of AEC. The truth is that on October 1st 1948 Associated Commercial Vehicles (ACV) was formed as a holding company upon the purchase of the Crossley & Maudslay concerns by the  Associated Equipment Company (AEC). Simultaneously, the manufacturing business, Associated Equipment Company (AEC), ceased to exist and became simply AEC Ltd that immediately became part of the ACV Group. In 1949 ACV took control of PRV and thus PRV became a sibling company to AEC Ltd under the same umbrella organisation.