The June 1968 Official Guide shows
Table d'Hote for A La Carte Service ... [the f looks like a typo -- and in the Official Guide!]
Iron Horse Tavern and Strata-Dome Lounge
Snacks and Beverages
(Pullman Lounge and Beverage Service -- Magazine Library, Tobacco and Travel Supplies)
By December 1970 it shows only
Iron Horse Tavern
Table d'Hote or A La Carte Meal Service
The Capitol Club
Lounge and Magazine Library
Snack and beverage service.
The last pre-Amtrak isssue, April 1971, shows the same thing.
So it looks like the dome came off sometime after June 1968, but it's not clear from the Guide whether they used a different car and continued to call it the Iron Horse Tavern -- some equipment expert will have to answer that.