Looking at the 1952 and 1953 Campy catalogues, there was a design change for both the bar-end shifters and the dt shifters. Perhaps the changer from bronze to aluminum levers occured in 1953? There was an accompanying change in parts numbers for the levers:
136 - lever for bar-end shifter (1952) 201 - lever for bar-end shifter (1953)
150 - right lever for dt shifter (1952) 601 - right lever for dt shifter (1953)
163 - left lever for dt shifter (1952) 602 - left lever for dt shifter (1953)
Aldo Ross
Monroe, Ohio