The Delta was introduced in 1964 and has been in continuous production ever since. Originally it was called the Davies Delta after the maker, and it was the first of the current line up to make an appearance. John, the current owner, is indeed the real deal and a hunting and killing machine. Last I talked to him he'd taken about 30 feral boars last year alone! He bought the line from Davies around 12 years ago and changed the name to Tusker. He's a working machine too, and he's made many improvements in that time, as well as having added several new heads.
Deltas are my favorite Tusker, they offer more cutting than the others and they fly exceptionally well for a large head. As Jerry mentioned, the steel in all Tuskers take and hold an excellent edge.