What's missing on this linkage?

Yes. hairpin style spring clip and a washer keeps the slider in place.
Spring goes over the outer groove.
Should be a good picture in the FSM - look in the Fuel Section for throttle linkage diagrams.
Another place that I *think* has good illustrations is in the MTSCs that review A/T transmission throttle linkage adjustment/
Go to www.imperialclub.org -> repair Lit -> Master Technicians Service Conf,
1971