These blisters are formed under the adductor of the left valve. They're not rare, in fact quite common across many molluscan species. They are largely idiopathic (unknown cause) however these present along the medial aspect of the muscle scar as opposed to entirely surrounding it, thus ruling out spondylitis. Outward along the same radius, there are tiny bumps near to the outer edge of the shell.
These are typical anomalies where damage or obstruction of the shell provided a point of entry for grains of sand and/or parasites.
The pallial mantle is well understood in pearl culture, where the vestibular mantle... not so much. This tiny ring shaped organ lays up the surface for new growth of the adductor muscle to attach. Hyper-extension and introduction of objects at the level of the VM most likely gave rise to internal extrapallial blisters (similar to mabe). At least one, but perhaps more erupted pearls were secondarily attached to the shell. Both points strongly support foreign body onset resulting from an environmental stress.
Not much monetary value here, but it's an interesting specimen nonetheless. Thank you for sharing it with us.