Here we have a DS4 first stage with a Tx50 second stage rigged on an 40 cuft deco bottle.
Notice that the 40" hose is routed to the left of the handle. That makes it easy to deploy and stow the reg on the bottle.
The 21 on the side is the maximum depth (MOD) for the bottle, 21 meter.
So there is probably 50% nitrox or 50/25 trimix in there.
There is also another 21 on the other side of the bottle so you can see the depth no matter how you hold the bottle.
As you can see I also have the second stage placed high.
If you carry many bottles it's usually better to have it lower on the bottle
since it has a tendency to be quite busy around your left d-ring otherwise.