You know that metal stamps can be imitated...so having one does not constitute absolute proof.
Never seen a Mikimoto piece stamped with a "K. Mikimoto" and "Tokyo" before but does not mean it is something impossible or that it never happened.
It also has a tiny "Mikimoto shell" stamp there, although it is not as well stamped as I usually see.
The pearl does look like an Akoya pearl. The setting? Maybe 1950's? 60's? Give me some Art Deco vibes.
Interesting piece!