For her 2000 book “Robert Indiana: Figures of Speech,” Susan Elizabeth Ryan, a Louisiana State University art history professor, spent considerable time interviewing the artist and digging into his process. She writes that “LOVE” began in late 1964 as a more explicit four-letter word — beginning with F, and with a second letter, a U, intriguingly tilted to the right. In the wake of a messy breakup with his on-again, off-again romantic partner and fellow artist Ellsworth Kelly, Mr. Indiana had been focusing on word paintings. The two men were in the habit of exchanging postcard-size sketches, with Mr. Kelly laying down fields of color and Mr. Indiana adding large words atop the abstractions.

“For Ellsworth it was like a joke,” explained Ms. Ryan, one that Mr. Kelly took as a provocation. Moreover, as a devout abstractionist, she said, “Ellsworth was horrified by the idea of having words in a painting. And the more he got like that, the more Robert wanted to take it seriously. During the era of post-painterly abstraction, just the fact that it was a word — any word — was subversive.”

The word in question was certainly in the air as a pointedly political gesture within the circles Mr. Indiana followed — from Ed Sanders’ similarly titled mimeographed poetry magazine to Lenny Bruce’s controversy-sparking performances.

Yet by December of that year, Mr. Indiana had shifted his artwork’s four letters to LOVE, using it for a set of handmade Christmas cards he mailed out to friends and shelving its blunter precursor. The director of the Museum of Modern Art saw one of the cards and asked Mr. Indiana to do a mass-produced version for the museum’s gift shop. By the end of 1965, MoMA’s “LOVE” card was a best-seller and unauthorized knockoffs were already appearing, including aluminum “LOVE” paperweights from the city of brotherly love, Philadelphia. Mr. Indiana doubled down, licensing his own “LOVE” jewelry, working on variants in painting form, and eventually moving off the canvas into three-dimensional versions.

This G-rated linguistic alteration wasn’t borne out of either self-censorship or marketing, said Barbara Haskell, a curator at the Whitney Museum of American Art and the organizer of its 2013 retrospective, “Robert Indiana: Beyond Love.” She ascribed it to Mr. Indiana’s canny aesthetic instincts, at least circa 1964: “Love has a bit more nuance to it than” its cruder four-letter cousin. That latter word “is a one-dimensional verb. It’s too flat-footed, it doesn’t leave anything to the imagination.” By contrast, “love is such a timeless, universal concept. Yet his graphic portrayal of it identified, on the one hand, a punchy affirmation of the term, and on the other hand, very deep feelings of fear. He saw it as a precarious image that came out of his disappointments in love — that tilted O suggests the instability of relationships.”