Hi There Bo!
I would venture a guess that the Silver/Grey (and I do believe they are Akoya) could possibly have been treated with Silver Nitrate? Almost every 'black' cultured Akoya pearl available on the international market pre-1960 were treated in this manner. The color is somewhat stable, and since the Silver salts settle in the conchiolin layers of the pearl, not adhere to the pearl's surface, the outer nacre layers of the Akoya will remain a semi-translucent white. The Japanese began using this technique extensively around 1930.
Have you had a chance to examine the pearls' drill shafts? The next time you have them strung, I would definitely take the opportunity to shine some strong light in that direction and see if the color of the nuclei might provide additional clues regarding treatment possibilities.
Additionally, the tooth test, while tried-and-true, is not 100% effective- cultured pearls that have undergone luster enhancements can throw you for a loop! The best course of action (other than contacting a lab), would be to peer down the shaft with a strong flashlight and at least 10x magnification.
If that is the original clasp, then I would definitely rule out any kind of Freshwater pearl- nucleation techniques were not that advanced back then! I also think that if the black pearls were irradiated, you might have noticed some color fade over the years...
That is an exceptionally beautiful necklace by the way!!
