Here is another example of the Publication getting it wrong.
The Alternator was changed to the 15ACR at engine 1000-2017. The service bulletin only quoted the engine number as the powerplant could be fitted to either the YG2S6 sedan and YJBAV6R van.
Attachment:
SLL 74 15ACR Alternator at 1000-2017.jpg
This change point was changed from engine 1000-2017 to car YG2S6 2017 in the PUB 1052 parts list.
Attachment:
1000-2017 Introduction of 15ACR alternator, YG2S6 2017 Mistake..jpg
As you can see I am very wary of using a Publication as a source of car number change points and total build numbers.
Now for the GT engine you have quoted. Using 1200-1500 as a compassion for 1200-1415 is inappropriate. 1200-1406 and 1200-1422 are June 1972 so that would also backup the theory that 1200-1415 is June 1972 as per the above document. Also YG2S8 1415 which is quoted in the above document is actually a Dec 1972 car and cant be listed in a June 1972 document. Perhaps one day you will believe me and my evidence.