For those of you looking for a quick reference for replacing those hard to find vintage seals try this web site. Once you find the stock number you can order them from your local auto parts store that carries Chicago Rawhide products.
Go to Industrial Applications
Then Online Catelog
Then Handbook of Seals
Then Parts Lookup
Enter the bore size in this format (##-##) being the "Min-Max" bore size when the little window comes up. If you want to see just 20mm shaft seals then type "20-20" etc.