What part is this grey keyhole shape and what set(s) is it from? I've recently found this grey keyhole-shaped piece - there are a fair number of them that stack together to form a circular staircase. There is what looks like a part number on the bottom but it is unreadable and faint. There is also a number beneath the part number, 201.

(images were made smaller but bigger ones can be added if needed).
Bricklink lists it as "Stairs Spiral Step" (Part# 40243). It first appeared in 2001 in the initial series of Harry Potter licensed sets.