Here are a few photos of the Plastimo 406 roller furler I fitted to my 19. I've only been able to take pics of the lower end without the sail attached, but I hope it gives an idea of the system.
The main part of the furler is a tubular aluminium extrusion that fits round the forestay, which must be a minimum 4mm diameter. This means replacing the original, but you'd have to do this anyway because it needs to be shorter and with a different swaged bottom fitting.
The head of the sail attaches to a swivel that is raised by the existing jib halyard, allowing the halyard to remain fixed while the sail rotates as it winds onto the furler tube. I also have a novel arrangement in that I've used an optional set of sliders that Plastimo supply, and instead of slliding the bolt-rope into the furler, I use the hanks on the sails to hank onto the sliders. This allows me to swap between jib and genoa as easily as with regular hank-on sails, but with the advantage of roller furling as well. I also remove the foresail when not sailing, so there's no need for a UV protection strip.