Until 1942 it was stated that 606 BCE was the date for the destruction of Jerusalem. Then someone realised adding 2520 years to 606 did not give 1914 (no zero year) and as they already had the answer they wanted from CTR's pyramidology they changed to 607 BCE.
This new date appeared in "The truth shall make you free" and further in "The kingdom is at hand" the following year.
All the old WT publications prior to this era use 606 BCE and of course 536 BCE for the return, this too had to be changed.
George