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Partial walk-around of the split part of the Y-ring.

Picture 1 of 3.

The straight leg of the Y-ring bolted to the adjoining part of the rocket, and the curved leg was welded to the propellant tank bulkhead.

The piece of metal in the gap between the two legs of the Y-ring would appear to be a "backup block", used in welding the Y-ring, which was formed by welding three 120°Y-ring segments into a ring. I do not believe that it would remain in the Y-ring after fabrication was completed.