These were very kindly donated by Dave Taylor of Devon; they're by Alan Puckett, who I must admit I've never heard of, but he can evidently draw motorcycles very well. The pictures here don't really do them justice - for some reason I found it much harder to get the colour right than when scanning photos. They're well worth a look, though.
Update - thanks to Murray Barnard for the following information:
Alan was born in Carolina USA in 1922. Alan has been drawing since the age of 7 and entered Yale Uni to begin formal training in art. He worked on industrial design before moving to Sydney NSW Australia to work as a freelance artist. He has a collection of antique cars and took up classic motorcycle racing with a passion. He had a beautiful Parilla 250 Sports road racer at one stage. The bikes you have featured appear in his classic work first published in 1977 - Classic Motorbikes by Alan Pukett with photography by Warren Penney. the book features wonderful paintings and photographs of some 30 machines.
And a further update - I've now been in touch with Alan, who tells me the pictures originally came from a book of his. Oops. Fortunately he's happy to have them on show here, and has kindly allowed me to update those which also exist on his site here, to improve the quality - and I've added a couple more. There are many others on Alan's site, but I don't feel I should copy any more of them; go have a look yourself, it's well worth it.