In old Triumph motorcycle parts books you will find part numbers listed with a letter prefix. The chart below shows the conversion from the old letter prefix to the new prefix number you will need to order your stock, vintage Triumph parts. For example, the part number E3974 converts to 70-3974.
In some cases you will have a part number that doesn't have four digits. For example, H519. You still convert the H to 97-, then just add a zero before the 519 to give you your full 4 digit part number, 97-0519.
You can learn more about Triumph engine frame serial numbers and get its identity for proper orders