The screw on the front is very short, only about 1/4" and seems to do nothing other than plug the hole it leaves when you remove it! I know that from yesterday, I was kneeling in front of the barrel, removed the screw, jiggled the tap and got a jet of water right at my crotch ending up resembling incontinence
Looking at the base of the tap I suspect the rotating centre comes out of the bottom but after removing the nut on the top I couldn't get the handle off or the centre to move even after tapping it with a mallet.
I doubt if its "O" rings, with the age of it more likely fibre washers, certainly that's what's on the screw at the front.
I don't really want to replace the tap, I've done a lot of searching and can't really find a quality replacement, just cheap plastic things that all get naff reviews.