On the reverse of the space before the second paragraph ‘Vehayah im shamo’a’ is the wordShaddai - meaning Almighty. The letters have come to stand for ‘Shomer D’latot Yisrael’ (Guardian of the doors of Israel) but this is unlikely to be it’s real origin.

shaddai close up

The Shaddai should be seen through a transparent section in the mezuzah casing, but since most mezuzot do not do this, the letter ‘Shin’ is often part of the outer casing design.