I agree that's what it looks like. Interestingly, during fabrication, the bulkhead was attached to the ring, and then the whole ring was lifted up and welded on

I agree that's what it looks like. Interestingly, during fabrication, the bulkhead was attached to the ring, and then the whole ring was lifted up and welded on. Maybe there was interaction of the heat affected area from the two welds? The bulkhead was welded only ~100mm up from the bottom of the ring.