Reading the code3M-300-9 is a 3M belt with a 300mm pitch length and a 9mm width. Some brands write it the other way round, such as 300-3M-9 or 9-3M-300, but the numbers mean the same thing.
Counting teethPitch length divided by 3mm gives the tooth count, so a 300mm 3M belt has 100 teeth. If the code has worn off, count the teeth and multiply by 3 for the length.
HTD or Supertorque?Supertorque S3M belts have the same 3mm pitch but a different tooth shape. They need their own pulleys, so check the tooth profile before swapping.