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).

Called it!
– rshea0 Mar 02 '12 at 02:43