I have to go to second hand information on this one, passed onto me by my Father, a jeweler from 1947 until the early 21st century. According to that, Granat Brothers was a manufacturer and retailer. They produced high quality die struck mountings. The retail end of the operation was acquired by Zale corporation in the 1960s, and the manufacturing end was acquired by someone else. This party continued to make mountings, without the Granat Brothers name, on sub-contracts with other jewelry firms. J. C. Keppie, a fine diamond firm specializing in Ideal Cut diamonds, sold some of these mountings in the 1970s and 1980s, but they bore the J. C. Keppie mark, not the Granat mark.
When Granat mountings were no longer available from Granat, Dad's store acquired the Jabel line, which they felt was comparable to Granat for high quality.
The retail operation was in San Francisco, and I think Zales made it part of their guild store division, along with Bailey, Banks, and Biddle and some of the other fine quality stores they acquired.
One thing I am sure of: When Dad got married in 1950 he went with a Granat Brothers mounting, because it was the finest available.
You may want to consider reusing the mounting or semi-mounting with sidestones, due to its high quality, perhaps mounting a colored gemstone in it and re-using the center diamond in a new setting/
Confession: During the high gold prices c. 1980, I melted about 100 old Granat mountings and bands, sure that those 1940s and 50s retro styles would never come back! Serious error in judgment there.