David, You may want to check out the listings under "tires" at Via Bicycle's website: http://www.bikeville.com
They have a listing for some Pneumatici Clément (Italia) tires in your size, 650a, for $75/pair. ~ Afraid they're not white, just gumwall, and I honestly can't tell what vintage...
HOWEVER, their tread design does appear to be very similar to my own Pirelli "Stella" tires... which are all white - and were first made in the 1950s, so... [Sorry, mine are 700x35c]
Good Luck!
Bob Hanson, Albuquerque, New Mexico, USA