Brief form for "upon"

The "word sign" for the word "upon" given in both the 1902 and the 1916 manuals is "pn."
In the Anniversary manual the brief from given for "upon" is "pon."
The Simplified manual reverts to the earlier brief form.
Does anyone have thoughts about the editors' reasons for changing the form in the Anniversary edition.  The original brief form is easier to write.

(by Bruce E. for group greggshorthand)