The only previous mention here seems to have been in the
Alan Mann book thread where, in response to a question about whether the book had any photos of the team transporters, Proviz posted this:
Just three, but one of them is of a transporter loaded with HWM-Alta about to set off to 1957 Naples GP and another a spledid colour shot of the ex-Reventlow transporter loaded with Cobra Daytona Coupes.
Bjorn, I think you should get this one anyway!
So it would appear it was
the Alan Mann. Perhaps someone with access to the book can provide more details.
As well as the 1957 Naples GP, the relevant Black Book has him racing in the first BRSCC Formula 1 Race at Brands Hatch on 14-Oct-56, also in an HWM. He didn’t have much luck in either race, retiring at Brands after 2 laps, cause unspecified, and at Naples with magneto trouble after 3 laps.