I think motoracer8 is correct, I have an old book published in 1974 called 'The Motorcycle Book' by Joe Scalzo, in it there is a story about Dave Aldana racing a Norton at Ascot, on this occasion he was riding for a guy called Harold Allison. I also have an article somewhere about the Woods Norton and I don't think Aldana was one of the riders.
It seems like in those days lots of riders would race freelance for independent tuners and share the prize money with them, so unless someone specifically remembers or trawls through old record books I suppose there is no way of knowing who rode what, the obvious exception being the works riders who are quite well documented.
I've heard that in a career some folks would ride for more than 20 owners. I used to know an old biker in Sonoma that had ridden for factory teams who had pictures of Indians and Harleys, as well as Triumphs on the wall. Now that I'm older I wish I had written down some of the things he told us.