Long Series 5230T.
Used for milling keyways and slots. Bright finish. Flute type: 30° helix. Manufactured to BS 122: Part 4: 1980.

METRIC TOLERANCES:
Diameter: ~ up to 3.00. Tolerance: +0.013 -0.033;
Diameter: Over 3.00 ~ up to 6.00. Tolerance:+ 0.020 -0.038;
Diameter: Over 6.00 ~ up to 10.00. Tolerance: +0.025 -0.046;
Diameter: Over 10.00 ~ up to 18.00. Tolerance: +0.033 -0.058;
Diameter: Over 18.00 ~ up to 30.00. Tolerance: +0.041 -0.071;
Diameter: Over 30.00 ~ . Tolerance: + 0.051 -0.089.

*Based on BS122: Part 4: 1980 though specific diameter is not included.