I've always hear it told as that he brought a box of drug store prints for him to look at and even purchased some for MOMA.
Way before the MOMA exhibition... from Wikipedia.
"Eggleston's development as a photographer seems to have taken place in relative isolation from other artists. In an interview, John Szarkowski of New York's Museum of Modern Art (MoMA) describes his first, 1969 encounter with the young Eggleston as being "absolutely out of the blue". After reviewing Eggleston's work (which he recalled as a suitcase full of "drugstore" color prints) Szarkowski prevailed upon the Photography Committee of MoMA to buy one of Eggleston's photographs."