The barrel has a small hole to the right of the key, to remove the barrel you need to turn the key all the way over to accessories and then put a small pin into the hole which when pushed will compress a small piston on a spring and allow you to pull the barrel free. Try the key at another position first so that when you turn it to accessories you'll have a feel for when you are actually pushing against the piston. With these old barrels there can sometimes be a little freeplay which will mean you'll miss the piston, keep trying and you'll find it.
