The rib extension is held on with solder and also a bind screw. The circle you see is the screw head. Every Parker gun made since the implementation of the dolls head extension (around serial 25,000) has it.
The screw is dressed off flush with the rib and matted over along with the rest of the rib.
Usually they end up being more visible like that if the barrels have been reblued, which is the case here.
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B. Dudley
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