Holland and later Dynamic Models made these glow plugs. They are sometimes available on EBay, once every couple of months. Also a head that was made to accept a standard short reach plug was made by Dynamic. The modified heads with a surface mounted glow element and the bottom plug seal seemed to give the best performance along with the original head/plug.
Doug Galbreath has a substitute head ring for the Hornets and will turn down standard Cox .049 plugs to fit it. Check his web-site and then e-mail him for prices. Just type “Doug Galbreath” into Google and it will take you where you want to go.