When you need to replace a hidden or “Butterfly” clasp, the first thing you will need to do is measure the clasp so that you can order the correct replacement.
To get a proper measurement, you will need:
A digital micrometer gauge or another measuring gauge that is accurate to one tenth of a millimeter
First you will need to open the watch clasp to access the end of the clasp where the measurement is taken. There are two places to give you a correct measurement, the outside of the end of the clasp, or the inside of the end of the band. (If the clasp pin is missing, you can order a replacement pin to repair it.)
Your measurement is rounded to the nearest half-a-millimeter (0.50mm)