The little triangular stamps impressed on the disk are inspectors' marks indicating the condition of the bore. One section of the disk, the smallest, contains just the numbers 1,2, and 3; that measures the surface condition of the bore in terms of wear, with no mark being best and 3 being worst.
The other, larger section of the disc indicates the actual diameter of the bore. There are two rows of numbers, one above the other. The bottom row starts with the number 6.46, followed by a 7 (meaning 6.47), an 8 (6.48), a 9 (6.49), and a 0 for 6.50. The top row begins with the number 6.51, followed by a 2 (6.52), then 3, 4, 5, 6, 7, 8, and finally 9 for 6.59. In the pic you can see the condition stamp at 1, and the bore diameter stamp at 6.50.