ACV Ltd (or Associated Commercial Vehicles) was an holding company formed on October 1st 1948 upon the purchase of the Crossley & Maudslay concerns by the  Associated Equipment Company (AEC)

Then the Associated Equipment Company (AEC) ceased to exist becoming AEC Limited that became part of the ACV Group and thus a sibling company to the Crossley & Maudslay companies that it had purchased.

The following year the ACV Group took control of Park Royal Vehicles Limited (PRV) [formerly Park Royal Coachworks (PRC) 1930-1946; from its formation out of the remnants of Hall Lewis] and its recently (1947) acquired subsidiary Charles H. Roe of Crossgates, Leeds.

Thus PRV also became a sibling company to AEC Limited, Crossley & Maudslay.  The Charles H. Roe concern remained as a subsidiary to PRV.

The ACV Group was merged with Leyland Motors in 1962 and three years later 30% of the PRV/Roe shares were sold to the Transport Holding Company (THC) for a stake-holding in Eastern Coachworks (ECW) & Bristol Commercial Vehicles.

In 1969 the nationalised THC became the National Bus Company (NBC) that later, along with British Leyland, created the non-trading Bus Manufacturers (Holdings) Limited as a jointly & equally owned enterprise that controlled the THC/NBC companies, the Leyland companies that included all that which formed the ACV Group.

And in 1982 Leyland took total control of Bus Manufacturers (Holdings) Limited.

Note: It's a false belief that PRV was a subsidiary of AEC. At no time was this the case; both PRV and AEC Limited were sibling companies under the same holding company ACV.