>From skimming some of cpython's "marshal" code [1] my best guess is that first difference is between it thinking the `_m` string might have another reference to it (and thus adding 0x80, or FLAG_REF to it) or not. This seems driven by whether or not python's object for the string has other references (it calls Py_REFCNT(v) to decide, see line 302).
I assume the difference is whether or not python has bothered to collect some other reference to the string or not. Type "Z" is an interned string type, TYPE_SHORT_ASCII_INTERNED, which therefore makes sense that it might be shared with who knows what else. I'm assuming this stops reproducing when you change it to a unique name since no one else will share the reference and you'll just deterministically get no FLAG_REF. Just my best guesses. Keith [1] https://github.com/python/cpython/blob/main/Python/marshal.c
