The 729 CE manual quotes 555 BPI. I’m not sure when it became 556. http://bitsavers.trailing-edge.com/pdf/ibm/magtape/729/223-6845_729_CEman_1959.pdf
The IBM 728 was 248 BPI. Before that it was the nice round number 200 BPI. I tried permutations of standard IPS and round number data rates and don’t see anything that yields exactly 556BPI. Tim
