It is all explained in the book you mentioned!
A good book it is, too.
You can catch people out with the question "which racing machine was the first to arrive on the I.O.M?" Of course, most people will plump for a bike!
My copy of this book is full of autographs of T.T. riders. I have some good ones, too.
I carry it around with me when I go watching racing, just in case I bump into a "blast from the past" The most recent entry is the late Peter Inchley...
Mike