Thanks Idolenz for sharing the information.
I updated the page accordingly: N#21454
Here is an interesting link: https://numismatica-italiana.lamoneta.it/moneta/W-GE31/30
Italian sources call them “zecchiere”.
If it means “mintmaster”, it's strange than some years have at least two of them. For example the initials IBI, IBM and OM appear at the same time from 1699 to 1705. I don't know their full name unfortunately. It's possible that IBI and IBM are the same person (similarly to ILM and GLM, which stand for the same person: Johannes/Giovanni Lucas Maiolus), but it seems less likely for OM.