James:
I'll photocopy and mail you the stuff I have on the Hughes S-43.
As RileyDove says, the 'Howard' in Howard 500 is Dee Howard, no connection with Hughes. Along with Bill Lear and Ed Swearingen, Dee Howard developed executive conversions of Lodestars and — in Howard's case — Venturas and one or two Harpoons. The pressurised, 350 mph Howard 500 like Duncan Baker's UK-based example was perhaps the pinnacle of piston-powered business transports, eclipsed by the arrival of the turboprop Gulfstream I. Some sources claim that the 500 was an entirely new-build aircraft with no Ventura components, and Dee Howard personally told me that this was so, but others dispute this.