Further information that will likely expand understanding on other additional causes of the wrong number punched on the warranty card can be found within this thread:
https://www.yenko.net/forum/showthread.php?t=169891
Additionally the maintenance manual shows the failure modes for the character set used on the warranty cards in 1968:
http://bitsavers.org/pdf/ibm/punched..._Man_Nov70.pdf
See pages 58, 59, and 60.