In some early masoretic Bible codices, a large letter resembling nun or zayin occurs in the margin, often in conjunction with the marking of qere/ketiv. Occurring in some codices, but not in others, the letter represents a bit of a cipher. Drawing on a database of ca. 15,000 masora parva notes, taken from 81 different classmarks, I propose that the letter, possibly a zayin, had (or acquired) a practical purpose, viz. as a means of avoiding certain types of copyist mistakes when recording qere/ketiv notes. Because the sign occurs in certain script types more than others, I also show that the notation can function as something of a regional identifier.