This is really going to get confusing figuring out the darn ring! Such a wide variation of possibility. Another poster was 115lbs in weight, and wore a size 8, because she had big hands. You were wearing a size 6 while preggers, and yet PB weight originally was estimated at 175 based on size 9 ring (but we don't know the finger it was on). Average unsized rings in jewelry stores/retail come a standard size 7. Depending on the ring design & stone settings, it can be sized up or down a max of 2 sizes, without reshaping/cutting the side shanks. I'm assuming the ring started out as a size 7, because the ring is still in a good round shape, and the shank designs don't look warped/stretched from sizing. It's easy to tell a ring that's gone to extreme sizing, I had a ring taken from a 7 down to a 4, and the bottom part of the band is somewhat oval, because if sides were pulled in to make it perfectly round, it would pop the stones out. Same thing in reverse, the sides of PB's ring would not be a perfect circle from the top of the ring, and look stretched. I don't see that in any view of her ring. Her ring was sized up twice according to the jeweler and going 1/2 size up is not that much of a difference. So, I'm pretty much convinced, the original size was 7, sized to 8, then 9.
ETA: If Balfours had records from 1974-75, we'd have a short list of possible owners real quick! Who bought size 7 rings with blue stones? But nooooo....that would be tooo easy!