The sprocket has a flat on it, which looks as if it is intended to suit a spanner. As Deejay said, if it screws off it must be on a left-hand thread or it would unscrew itself in service. Reel sprockets sometimes are designed to unscrew clockwise while a piece of soft wood is threaded through the reel itself to prevent rotation. However the Model 17 was a very classy machine, and I would not be surprised if it had a better way to restrain the reel, such as a hexagon or flats on the reel's axle, or a place to slide a pin through both axle and bearing housing. A picture or two would be useful, showing what is right behind the sprocket, and the reel axle just inside the sprocket-side bearing. If there is nothing there to lock the reel, chances are the block of softwood through the blades is the answer.
