The change was in 1960 production but not at the start of the production year. On page 255 of Noland's book he states that the AIM calling for the addition of the serial number to the stamp was dated 3-28-60. However there are earlier examples known. The earliest he knew of at that time was serial number 1806 assembled on the morning of Friday, December 18th, 1959.
Another early example is car number 3679 built early in the morning of Friday February 12th 1960. The date that the AIM update was released to the line and the date that the practice actually started seem to be quite different. I can't say for sure that all blocks were stamped once the practice started and before the AIM update reached the line but I'd say it would be a good guess. Also I think there could be blocks stamped with serial numbers closely before car number 1806.
I don't have a copy of the latest judging manual so maybe someone will add more to this discussion that has had recent experience on the matter.
Tom