I have a stock of many of these and am happy to have a play day at Haddo, if Mike is happy. I am basically waterproof and, with a standby diver for safety would happily test each type. We could try a number of styles and test them upright and inverted. I would have to wear a dry suit, but can ge one with no added buoyancy.
I agree with H, in that that these are buoyancy aids, but this is so much better for the consious casualty. After testing many life jackets in a pool the hazard is that if the water is less than 5-6 feet deep they will not turn you face up, nothing will, as your weight and drag on the bottom will overcome the buoyancy of a life jacket.
To sumarise, in shallow water jackets will hold you up, but can hold you face down, indeed can trap you this way. In deep water a life jacket will hold you face up, and a buoyancy aid will hold you head up. In my opinion, selling and testing these in the water for 20 years a buoyancy aid is well suited to shallow waters, rivers and lochs. A life jacket is vital for deeper and open waters, the sea and deeper or larger lochs.
D