The green rod is the new factory plug Remington includes when you buy an 870, at least when you buy an express.
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Police and Wingmaster as well ...
I thought that might be the case, I wonder if they switched over to the new style plug when they went with the new spring retainer and dimples? The plastic retainer is thicker than the old press in metal spring retainer iirc, I bet the old green plug can’t fit inside the new plastic retainer.