Russ,
One way to tell is to look at the frame. Factory vent rib guns are flat across the top of the frame where the dolls head is. A gun that was sent back to the factory for a vent rib may not be flat though. I would also think the rib matting and "Parker Special" engraving would tell you a lot. I think the Parker legend on the rib was engraved instead of roll stamped.
Patrick
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