This 751.2 is a reamer for the HTR, and it is sized to 30.1mm. The allows a 0.1mm interference fit when pressing the 30.2mm headset of the "euro" one-inch standard.
NOTE: Always use a cutting fluid such as the Park Tool CF-2 when machining.
SHIS terminology: The 751.2 is used on EC30 headtubes. If used on the EC29 headtube, it will ream it up to the EC30
Manufacturer's Part Number: 751.2