Although the timeline above seems correct, previous to the CE's there were service engines w/ X code Suffixes like XE and others..
I may be off here but believe X code engines had specific HP ratings more like buying an over the counter standard Suffix engine minus Vin where CE's had unique individual #'s that were recorded as supplied to specific Vins under warranty?.
Fran Preve @ Tonawanda told me in the 80's the books w/ the CE recorded info were tossed or lost..
~ Pete