According to the shop manual supplement dated 4/1959, issued for the change to the split case transmission, the chassis numbers for the changes were: Berlina 1488.20801, t.i. 1468.8501, Sprint 1493.7301, and Spider 1495.4811. I don't know the dates corresponding to the chassis numbers, but it is around September 1958, as Greig says. My January 1959 Spider is 1495.05875.
Alfa dropped the place holder zero in listing chassis numbers for the t.i., Sprint and Spider in this shop manual supplement.