omniglot.com/language/numbers/oldwelsh.htm - Old Welsh numbers.
omniglot.com/language/numbers/middlewelsh.htm - Middle Welsh numbers.
Gramadeg Cymraeg Canol, by Simon Evans (1951)
A Grammar of Middle Welsh, by Simon Evans (1964)
Old and Middle Welsh, in The Celtic Languages, 2nd edition, by Willis (2009)