Westley Richards made not only single trigger rifles they even built U/O rifles in 318 and 425. One of Christopher Austyn's books goes into detail about them, I am not sure which of his books though. PM J Tomlinson here - he would know about these in detail, if anyone does. You'll also make a good friend in a very short time by communicating with him. He is one of the nicest persons I have had the fortune to befriend on any forums.

Good hunting!