I guess you's call it "custom". It's made from the original head. It was possibly made by the well know Australian tuner, Stan Pilgrim. Stan is one of the "Motor Boys" a group that will be well known to those more interested in the technical side of model engine construction. I believe that Stan may have originated the notion of this type of 2-piece head. Gordon Burford copied this idea in the batch of 200 (or so) Goldhead 15 FIRE diesels he produced in the 70's.

Ray

PS Actually it may have been a batch of 100 (or so) Goldhead diesels