Usually I will share the original image and text from my source, but I fear, all data of my puzzles are lost forever, due to a HD-crash. I rather know, that I've found the picture of the car in a British Classic Car Magazine, 'Classic Cars' or 'Classic and Sports Cars'. I fear, I or the authors of the article mixed up something with "GRP Massey Bros."
"GRP Massey" mainly built fridge-superstructures. I ignored "GRP" and had an eye on "Massey Bros."
But where was "GRP Massey" located and when founded?
Robert B. Massey & Co Ltd was founded in 1946 and grew to include several dealerships, coachbuilding facilities, and other automotive businesses at various locations in and around Yorkshire (e.g., Hull, Beverly, York, Market Weighton, etc.). The company expanded over the years, purchasing other businesses and creating subsidiary businesses, and at some point, the name of the parent company was changed to "Robert B. Massey Group Ltd." A number of these subsidiary business were eventually sold off or closed, but at least one of the company's dealerships, an East Yorkshire Range Rover franchise, is still operating under the name "Armstrong Massey" today.
GRP Cooltrucks Ltd was founded in Leeds, West Yorkshire in 1980. The company changed its name to "GRP Massey Ltd" in 1988, then to "GRP Leeds Ltd" in 1998, and finally to "GRP Enterprises Ltd" in 2011. It was dissolved in 2015.
I haven't looked into it, but the Massey Group may have purchased GRP Cooltrucks in 1988 and sold it in 1998, which would account for use of the Massey name during that period. And if your source magazine was published during that period, then it makes sense that the author may have used the name of one of Massey's existing businesses when referring to the coachbuilder.
Was the rebody performed in 1954? If so, that puts it more-or-less in line with the period in which Robert B. Massey & Co Ltd created the estate/woody coachwork for the Rileys above (1946-1950).
(All of this is completely unrelated to Massey Bros in Wigan, of course.)