A small shipment of springs has arrived from the local distributor of Century Springs, so spent some time playing with them.
I got Stock number 1887 for the rudder cable 'tailplane' springs, as called out in the Hidden Rudder Bellhorn Plans. They seem rather stiff when mated up with my hooks. My motorcycle chain outer plate joining bits seem to work fine.
I got stock number 5688 for the rudder return springs, and reckon they will serve nicely.