The breaking down of the dies on the small dates created the need for the large dates. The problem with the chips/cracks and other issues could not be tolerated. So they came out with the large dates. I've heard it mentioned that this was done quickly and that the issues with the large numbers of RPMs that year were caused by because this rush to get these dies ready for use. (but the 1961 year also had issues with a lot of RPMs also)