Vintage Mikimoto Pearls

Hi, in researching a strand of Mikimoto Pearls I purchased came across your post. I have had a couple Mikimoto Strands (authentic) and a couple Mikimoto Clasps so I am a little familiar with the Mikimoto Hallmarks, etc.
The strand I purchased has the same Clasp as yours and the same type of box, my box is silk embroidered on the outside and no Japanese characters. My Clasp is Sterling.

1) If you use a loupe you will probably find on the catch portion of the clasp a very small Mikimoto "M in the Clam Shell" which is a bit distorted because of it's minute size. Mine is on the end that connects to the jump ring.
2) On the side of the Ornate portion of the clasp you may find what I have "Sterling and the M with a portion of the Clam Shell" again a minute area to Hallmark so it's very hard to see without the loupe.

I hope this helps!