Hi Gus, as you can see, on your mower the reel sprocket's retaining nut is welded to the sprocket. That means the sprocket has to screw onto the reel - it can't be keyed to the reel's axle. Note that the engine output shaft, which attaches to the top sprocket, rotates anticlockwise viewed from the chaincase end. Therefore the reel also rotates anticlockwise. If the reel sprocket's thread were a normal right hand one, it would unscrew itself when the reel was under load cutting grass, so it has to be a left hand thread.
To remove the reel sprocket, remove the chain, and put a piece of steel bar of the right diameter through the holes in the collar behind the sprocket, to keep the reel from rotating. (On most mowers you have to put a wide piece of wood through the reel to prevent it rotating, but yours seems to be a more luxurious version.) Unscrew the sprocket retaining nut by using a socket spanner and rotating it clockwise, as if you were tightening a normal right hand thread. The sprocket and nut will screw off together.
As a point of good workshop practice, the collar under the reel sprocket seems to be designed to be held by a C spanner when unscrewing the sprocket. The C spanner would presumably have been a special tool available from Scott Bonnar. I tend to make special C spanners when I expect to use them more than once, not least because if you just use a steel bar, it mashes the initially-round hole in the collar and makes it look like the tenants have been playing with it.
