There is a nipple on the other side of the stock. And to be honest the thumb knob rubs on my hand so I took it out of the stock. The previous owner did the mod, and to be honest it is a very good idea, and very thoroughly thought out.
You can see a little blue tube that is hanging just by the mag well.... that is what plugs into the mag, they are taped on the side of the mag with a push type connection. The tube runs below the trigger guard, and then back to the internal reg and on/off switch.