If you show us a picture of the Battery Compartment (or the backside of watch), someone might be able to better guess the battery size.
I looked up Concord Watch info from the '70s, and this is what I found:
CONCORD -
(North American Watch Corp.)
1345 Avenue of the Americas
New York, NY 10019
(212)-586-6569
...(I'm sure this Watch Company does not exist any more, and the phone number none working...This info is really super old).
Maybe this helps a little?
Good Luck
