The '68 had a vacuum retard as part of the emissions changes for that year. It connects to one of the five ports on the TVS. A tech bulletin (68-T-2) removed it on many V8's. To date, after a lot of studying it, I still don't know if 400s were meant to be included on the bulletin or not but it was removed by Mr. Goodwrench on my '68 400HO way back in the early '70s. In '69 the TVS lost two ports.

Edit: Yes, according to the '68 Diagnosis manual, it was said to be there to reduce timing on idle and "coast down" to reduce emissions.

(I have been thinking of adding "dummy" striped hoses back to the removed TVS hoses for the original appearance, thinking the 400s were not supposed to be part of the bulletin.)

Looking at those parts diagrams, it appears to be something associated with the '67 California AIR equipped cars which joseph67 mentions.