Originally Posted by Andrew R Stewart

Detach the cable from the rear der. Using your fingers move the der across it's range and thus shifting the chain from cog to cog. Using the limit adjusters prevent the der from moving too far to derail the chain. Now reattach the cable. This is just like what is done with any der system. The only difference is that modern ders might not have enough limiting ability as modern stuff is designed to be used with modern stuff (and no modern FW that I know of has only 3 cogs). To be specific- use the der's built in limit features and not the cable to control how far the der can move. Never rely only on the cable to do this.

It is quite possible that the frewheel was at one time a 5 cog one. I've seem freewheels with missing cogs (hardened steel can crack) and various "spacers" added if needed to continue the function with the missing cog. Andy