I was browsing through the 1963 edition of BASIC SCUBA, by Fred Roberts, and for the first time noticed his section on "Home Built Lungs and Equipment" beginning on page 283. Understandably, he discourages conversion of the readily available (at that time) military surplus aviation Oxygen Diluter regulator into diving lungs. Here's a couple of excerpts:
I personally know a number of divers who began their careers with a converted Air Force diluter regulator, and several of the conversions were quite thorough and well planned. However, not one of those divers would trust his unit in water over 30 feet deep, and all but one experienced a failure.
No other unit, or device, in diving has caused more deaths in the course of equipment failures than the home built lungs and surplus apparatus in the hands of a novice.
Unfortunately surplus outlets still have such units available, often in kit form.... They appear safe because of unscrupulous false advertising, and most of the time the people selling them do not know how to use a lung....