"Chess" of "Bechozek":
A few months ago I had a similiar "Chess" and Rabbi Shamai Gross passeled it Because the left "Zayin" is shaped like a "Nun" which makes it a "Tof". Recently, When I was checking this guy's Tefillin I noticed a similiar "Chess" and I brought it to Rabbi Wosner in Montreal (the author of Shu"t Chayei Halevi, nephew of Shevet Halevi) and he said after much thought "S'is gornisht"
Here's a copy of Tefillin which are Meyuchas to R' Reuven Sofer, the source of Ksav Chabad. Notice the "Chess".