There is a tradition that the Alter Rebbe (R' Shneur Zalman of Liadi) innovated a new Ksav (not the one in the Shulchan Aruch Harav) and taught it to sofrim who passed it on from teacher to student.
According to R' Zilbershtrom, the Lubavitcher Rebbe couldn't find a reliable tradition of this script, and didn't use it (According to him, both the Rebbe and the Previous Rebbe used Arizal teffilin). Therefore, he says that the "Alter Rebbe Ksav" that exists nowadays is not the real ksav.
We have a Megilla of the Rebbe Maharash (the fourth Lubavitcher Rebbe).
Why can't we just compare our "Alter Rebbe Ksav" to that Megilla?