Concerning the practice of women having their heads shaved, please refer to the following link:
http://hebrewbooks.org/pdfpager.aspx?req=46546&st=&pgnum=48
However the Mechaber of the sefer does not differentiate about different groups holding by this practice and it’s not clear how widespread it is.
Even the Chasam Sofer earlier toys with the idea of the practice being midat chassidut but is hardpressed to consider it normative halacha.
old man — The following Teshuva by Rav Menachem Klein appears to validate the origin of the Takana, but goes further to state earlier origins. In his Teshuva, he does appear to use the word trim and not necessarily shave.
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Sof kol sof, I am not paskening normative halacha, simply showing the origins of this practice.
As to why your copy of the Pinkus did not seem to state this Takana, I am not sure. Tzarich Iyyun.