In anticipation of the OP returning to action, I thought I would let him know what very little I had found. There is a forum of which I am an inactive member, which comprises a number of very well-informed and knowledgeable motor-sport historians. There is an old thread on that site about Maserati 8CM cars, and also a sort thread about what is commonly known as "the Finglass car". I resurrected that thread to ask questions about our puzzle photo, which is why it would be good to know the date of the advert. I centred my enquiry around the fact that the car was being sold by Finglass in 1955, but our puzzle vendor is a Mr. Salmon.
I received only one reply on the thread, but it WAS from a well-known and respected author of Maserati-themed books. He advised me that, to the best of his knowledge, the conversion to road car had been started by Salmon and finished by Finglass. That is all, as they say. Still no nearer to finding out who did the bodywork, but personally I'm convinced it wasn't done by an established "name" in coachbuilding, but rather it was farmed out to a local bodyshop.
I thought you might like to see the car as it is today. Bear in mind that it was delivered to Earl Howe as a "wide" 2-seater chassis, and presumably painted by him in his regular racing colours of pale blue and silver. It has been "restored" as a single-seater and painted a very dark blue. I suppose the meaning of restoration has very wide parameters.