Please explain how to decrypt Plomb date code stamps

[Stevenn1]

I looked all over and can’t find info. For example, a date code stamp of 7B or whatever. Is there a list of their date codes somewhere?

 

[Stevenn1]

I am still searching but the only thing I can come up with is this from the website:
http://www.vannattabros.com/plomb/plombtool.html