There is a product number on the D-M9B auto-switch part and after this there is 3 alphabetical letters

The 3 letters indicate the production lot number, the year, and the month of production. This is not related to the auto-switch number.
Note 1) 2 letters shown on auto switches are also the production lot number.
Note 2) In some cases, letters shown in a square.
Note 3) The production lot number for the reed auto-switch is to be indicated with numbers.