Snap on 3/8" Drive 6-Point Metric 18 mm Shallow Impact Socket IMFM18.  Used in good shape. 

 Flank Drive® wrenching system delivers up to 20% more turning power without slipping or rounding while exceeding ANSI® performance standards

• The hex depth of our impact sockets will start to thread the nut onto the stud; competitive sockets with a deeper hex require the user to start the nut by hand

• Snap-on® impact sockets are heat-treated to optimize impact service life with a lower hardness than hand sockets and are able to withstand repeated cycling loads of impact wrenches

• Manufactured from the highest quality proprietary steel, providing optimum strength, durability, and longer life

• Square drive side hole works with ball- or pin-type retention

• Industrial finish helps protect against corrosion