The large hole in the wrench is internally D-Shaped to conveniently hold the small end of the piston from rotating during cleaning.
The Steel model is CNC machined from 1/4 thick steel with a smaller beveled edge. Designed to fit all original USGI and most commercial gas cylinders. All tools are finished with a baked-on Non-Mar Mil Spec coating (same spec used by the military) in your choice of colors. This finish is .002 - .004 thick and very durable, plus, the wrench won't damage or mark your gas cylinder.