I was recently shown a very old wooden device, apparently intended to illustrate or aid in mastering some arithmetic procedures. It is about 2 ft tall by 2.5 ft wide. There are 9 slats which slide horizontally. Each slat contains 10 digits, although only 8 of the digits are visible at one time. Some slats contain all digits from 0 to 9. Others contain only some digits (e.g. 0,1,2; 6,7,8,9). There are also cloth strips, hanging from the top like narrow window shades, each wide enough to hide all or part (depending on how far down you pull the "window shade") of one column of the visible digits.
I can't figure out what it is supposed to do. Any ideas?