I have now found the "M2" mark, but I think that the "owner" of it, was a sub-contractor to Hoijkaas/Hooykaas. The mark I found is on a three piece art deco tea set designed by Harm Ellens.
I cite the description by an owner of a website without violating the rules of this forum, I hope... I think you will find it and will be able to have a look.
"The hallmarks include makers mark HH, Lion Passant for 833 standard silver, (with keymark indicating it was made for export), Perseus (sic) duty mark with mint mark M on helmet, indicating Schoonhoven assay office, date letter M for 1922 (source Tardy, International hallmarks). The base of each also has the hallmark M2 and a scratched number 42299."