Thanks for the feedback. I have learnt from Ron Stein (TE/AE Lake Ontario rep) that B94103022 (early March 1965) was the introduction of the square doors and hoods. The last Series IVa was B94104470, so only 1448 were produced with square doors and hoods.

From Victoria British:
Series 4 1592 cc Jan 1964 – Sep 1964 B9400001 up to B9407936 Total made 7,936
Series 4a 1592 cc Sep 1964 – Sep 1965 B94100001 up to B94104470 Total made 4,470