Shir HaMaalos Question

Today, I was browsing through an auction site for seforim and came across this.

(Photograph courtesy of Virtual Judaica)

In the description it says:

Kabbalistic Shemira poster normally placed by the crib of newborns, or according to other customs above the bed of the woman and above the doorposts of the room. The Shemira poster is also known by other names, such as kindbet, tsetl, and Shmir-Tsetl. In all cases its purpose is to protect the baby from Lilith or other harmful spirits. The name comes from Psalm 121, which here appears below the header, Shemira le-yeled ve-yaldah. That Psalm begins, "A Song of Maalot. I will lift up my eyes to the mountains. From where does my help come? My help comes from the Lord, who made heaven and earth. He will not let your foot be moved; he who watches you will not slumber. Behold, he who watches Israel shall neither slumber nor sleep." The Shemira is erroneously attributed under the title to R. Israel Ba'al Shem Tov (Besht).

I have always heard that putting this sign up near a baby's crib was a segula that was attributed to the Baal Shem Tov.

Does anybody know anything about this?

UPDATE: Another Shir Hamaalos sign can be seen here. Attribution to the Baal Shem Tov is mentioned on the back here.

Note: The sefer "Segulos HaBesht v'Talmidav" does not mention this segula or attribute it to the Baal Shem Tov.