I don't know when the heaters switched from the thick to the thinner cores. I'd be guessing that this happen in the 1974 model year when the GTV2000's came out?
FOR THOSE WHO CARE; Ross and I discussed the heater core thickness issue today and Ross concluded that the thick cores had straight fins and the thin cores had wiggly fins. The fins can be seen by looking in the top big intake when the flapper door is open.
This was born out by my collection of heaters that has both thick and thin cores but I could not relate either type to particular years. One I found with thin core was from a 1300 1971 GTj. My 1975 1600 GTj has a thick core. I wonder if the heater selection was related to intended market. Both type were made by ISPRA.