The bolts are graded in diameter for a more difficult exercise. Five slotted bolts and stubby screwdriver. Designed by a Montessori teacher to aid the development of eye-hand coordination and teach a practical skill.
I published a post about inexpensive and DIY sandpaper letters, with many ideas that work for sandpaper numerals as well as letters. Here I'll give resources specifically for sandpaper numerals and sandpaper-numeral presentations and extensions.