Looking in the 1932 and 1933 Excelsior catalogues there are similar 500cc bikes, the B14 and C14. These have single port special racing engines, burman 3 speed close ratio boxes and webb racing forks with hand operated shock absorbers. Max speed in the region of 100mph.
I have a 500cc Excelsior J.A.P project, which I am fairly certain is a works experimental bike raced in the 1932 Senior TT. If you come across any information on these bikes I would like to hear from you, as ten months of searching has turned up very little information.
DM