A tuna fish sandwich (or a tuna salad sandwich) on toast is called a “radio” (or “radio down") in diner slang. “Tuna” was taken for “tune it,” and “tuna down” meant “tune it (the radio) down.” The slang seems to have existed in the 1950s, but died out by the late 1960s (along with many old radios).
March 1960, Fast Food, pg. 40, col. 1:
By Harry Press
I entered pharmacy about 39 years ago. The first store I worked in had a soda fountain, and I fell in love with that end of the business.
In those days, soda fountain workers had a language all their own. In order to save (Col. 2—ed.) time, or just to be different, they used abbreviations or code words, or numbers for almost everything they ordered.
Some of these were quite clever, but some were often embarrassing to the customers. For this reason, most chain (Col. 3—ed.) stores and a great many independents have stopped using them.
Here are a few of the “codes” still used behind some fountains.
Radio—tuna fish sandwich.
Radio down—tuna on toast.
22 June 1960, Raleigh Register (Beckley, WV), “CB, Rye, Hold the B, Is Vanishing Jargon” by C. J. McQuade, pg. 4, col. 7:
In one particularly creative establishment, the evolution of a toasted-tuna-sandwich lingo went like this: from tuna down to tune down, an eventually to radio!
21 May 1966, Newark (OH) Advocate, “Railroads Have Slanguage,” pg. 3, cols. 3-4:
Short order cooks and waitresses in the nation’s diners also have their own slanguage. It’s designed to save time and avoid confusion. Here are some good examples of their shorthand vocabulary:
radio—tuna fish sandwich on toasted white bread;
5 September 1971, New York (NY) Times, “The Traveler’s Guide To Hash-House Greek” by Dan Carlinsky, pg. XX7:
radio. Tuna sandwich on toast. Derives from “tune ‘er down.”
Google News Archive
12 December 1977, Village Voice (NY), “Counter Code,” pg. 60, col. 6:
Radio: Tuna. A New Yorker’s version is often tuner. Hence, radio.
Listening to America:
An illustrated history of words and phrases from our lively and splendid past
By Stuart Berg Flexner
New York, NY: Simon and Schuster
radio, a tuna fish salad sandwich on toast (a “tuna down,” punning on “tune it down”; see down above).
The Dictionary of American Food and Drink
By John F. Mariani
Published by Ticknor & Fields
radio. A tuna fish salad sandwich on toast, punning on “tuna down,” which sounds like “turn it down,” as one would the radio knob.
Sundae Best: a history of soda fountains
By Anne Cooper Funderburg
Published by Popular Press
Pg. 171 (Hash-House Greek):
RADIO: Tuna salad sandwich made with toasted bread.