I found a McCoy Baby-Mac Hot Point Glow Plug with box.
The small "Baby-Mac" glow plugs were designed for the converted small McCoy engines at the time. They had a screw on cylinder cooling jacket, so they drilled a hole and machined a step or shoulder to hold the glow plug in when the cylinder jacket was screwed on. Towards the end of the McCoy Duro Glo diesel engines, they redesigned the cylinder and cooling jacket to use a glow plug insert. The cooling jacket held the glowplug in place. This was on the mcCoy .049D model engine. The cylinder and cooling jacket were not interchangeable with the regular .049 Duro Glo model diesel engines. Ref http://www.modelenginenews.org/ad/mccoy_diesels.html