Mike Bullington, senior archives manager for McDonald's Corp., MCD +0.71% is a connoisseur of fast-food artifacts.

At work, he's surrounded by bags and packaging from years past. His cubicle holds a purse woven from Big Mac wrappers, a gift from a former chairman's wife, and a Ronald McDonald fashioned from Legos, turned in by a McDonald's franchisee. But the prize he treasures most recalls a menu item that had a brief stint in restaurants: Onion Nuggets, a precursor of sorts to Chicken McNuggets.

"Not many people have seen them, even within the company," says Mr. Bullington. The packaging, he says, is a "hidden gem."

The batter-fried lumps of chopped onion themselves are lost to history. But Mr. Bullington has preserved a sample of the late-1970s paper onion vessel in a climate-controlled room in the former Hamburger University building in Elk Grove Village, Ill....