GAUGE FOR CLOCK KEYS 
Solves the problem of how to measure the square hole in a clock key.
Place the clock key over the square on the metal plate to obtain the correct key size.
It has 18 sizes of squares rods
THE KEY SIZES ARE
Size 3/0, 1.75 mm ,Size 2/0, 2 mm ,Size 0, 2.25 mm ,Size 1, 2.5 mm ,Size 2, 2.75 mm ,Size 3, 3 mm ,Size 4, 3.25 mm ,Size 5, 3.5 mm ,Size 6, 3.75 mm ,Size 7, 4 mm ,Size 8, 4.25 mm ,Size 9, 4.5 mm ,Size 10, 4.75 mm ,Size 11, 5 mm ,Size 12, 5.25 mm ,Size 13, 5.5 mm ,Size 14, 5.75 mm ,Size 15, 6 mm."