The Standard Railway Wagon Co Heywood Works book by David W Tandy ISBN 9780955235429, attributes the build to Silcock & Colling / Railease.
"They were converted at SRW Heywood and Reddish Works."
"This fleet was originally designed for the carriage of vans, Range Rover and Land Rover vehicles, too big to be carried on Cartic type wagons"
"They also carried Vauxhalls and Renaults spending most of their life working to and from Perth, and Bathgate to Garston and Dagenham Docks"