New Square Gauge Clock Key. This is a new square gauge clock key. This 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. Assures accuracy in ordering the right key. It has 18 sizes of squares. The 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.