When I first started diving in the UK most UK divers where on single cylinders, 12l or 15l. My club was slightly unusual, because a large proportion of the club also carried 3l pony bottles as bailout cylinders.
At the time we all had the bailout (pony's) valve up.
Later I had switched to a twinset. A few years after this, I completed one of the few Nitrox (or as they where known at the time, devil gas,) courses available in the UK. Oxygen was very difficult to find. So we tended to use a 50% as the deco gas [1], carry air in the twinset and run the dive as if we where on air for the whole dive. Deco bottles where generally 3l pony's striped valve DOWN to the side of the twinset. Valve down gave us access to the valve. We could charge the first stage and then isolate the gas by turning the cylinder on. Then as you reached 20m you switched the deco cylinder on and switched to the deco gas.
[1] A 3l would do a weekend, used for the first dive on day 1 dive 1 (the deep dive). Used again for dive 1 day 2, and then, if anything was left used on dive 2 day 2. It also meant we could change sites without depth restrictions, and not have issues getting Nitrox as the bottom gas.
