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"U" and "U#" formats were introduced by r55433 (Python3). At this same, "s" and "U" formats were different: "s" called PyString_FromStringAndSize() and "U" called PyUnicode_FromStringAndSize(). Two months later, PyString_FromStringAndSize() was replaced by PyUnicode_FromStringAndSize() for format "s", and so both formats are exactly the same.
"U" and "U#" were introduced during the transition between bytes, str and unicode. They can now be replaced by "s" and "s#", and then be removed.
There is just one usage of "U": definition of sys.subversion (Python/sysmodule.c), whereas there are 36 usages of "s" format. (U# and s# are not used.)