I don't believe those "experimental" pearls could have looked like these. If this is an original Mikimoto brooch it probably had Akoya pearls that were later exchanged.
A colleague of Mikimoto's (Dr. Fujita) started cultivating freshwater pearls in Lake Biwa shortly after Mikimoto began culturing whole pearls. They used freshwater mussels and some of the pearls were sold in Mikimoto's store. The pearls were non-white and baroque. They were not smooth drops. The drops in this brooch look like very typical Chinese freshwater pearls.