Gil,
Re conventional doors on 600's:
Madaro, in his book Fiat 600 and Multipla (Giorgio Nada Editore), says conventional doors commenced "dal Maggio del 1964, dalla vettura n. 1,821,001..... (loosely..from May 1964, from body number 1,821,001) . Introduction in various markets probably occured some months later where earlier assembly kits were still in stock.
Trevor