I do not know the answer to your question... we have a 1963
Bambi and our fresh water is around 20 - 25 gallons (we have the original galvanized tank) and our tiny black tank is about 7 - 8 gallons... you might check the vintage airstream thread and see if any information is here...
Vintage Airstream Photo Archives
Otherwise, I would do a search for 1964
Globetrotter fresh water tank and see if you find anything. Somewhere there is a listing of owner's manuals... I don't know if they were producing them yet in 1964
, I know they did not for 1963
.... you might check that in a search as well. Good luck!
Mrs. NorCal Bambi (traveling in S Tardis ~ from the Great State of Jefferson)