The item shown appears to be a ABLJ (Adjustable Buoyancy Life Jacket), the forerunner of the modern Buoyancy Control Device (BCD). The attached air bottle is not primarily aimed at providing breathing air but for inflating the jacket in an emergency to keep the diver on the surface. There is a technique where a diver can use the very limited air supply provided by the bottle to make an emergency ascent but it takes training to use it. The cylinders were known in the diving world as 'suicide bottles' because opening them at depth often led to an uncontrolled ascent with the attendant risk of decompression sickness and/or burst lungs.
