I believe the white strand is an Akoya strand...could be from Mikimoto: its near impossible to actually tell pearls from a "brand perspective" unless you have a certificate, a receipt, the clasp or any other telltales that scream "Brand Name", and even so: you can place pearls in a different box, give someone a receipt for another good, change the clasp or even the pearls.
So, unless you have someone telling you: Yes, I got this item at a Mikimoto shop in Ginza back in 1975 and I have never re-worked it in anyway...then you can be sure(r).
I think the necklace is very nice, and those "black pearls" are really nice looking. The combination works great.