I put this on my RM Series Facebook page and got he following reply from Ros King "It will be the conversions done from "Donald Healey Motor Company". He made the Healey-Silverstone using the 2½ Riley engine and also 'upgraded' a few saloons. . . a posher version of the Downton Heads/Conversions for mini's...... My supposition as I have no data this end on them easily to hand"
There is a guy in the Milton Keynes area called Warren Kennedy who runs a restoration business for Healey's, MG's etc. I feel certain he would know a great deal on these modifications.