UNC 5/16" Socket Screw Countersunk High Tensile 10.9 DIN7991 UNC 5/16" (18 T.P.I.), Coarse Pitch, High Tensile (Grade 'S'/ 10.9), Self Colour. This space of unthreaded area confirms that the fastener is now a bolt (socket countersunk bolt in this case) and is shouldered.
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