This was interesting, though print research might not be as useful as we'd like. As "slush pile" is primarily professional jargon, spoken between members of a publishing staff rather than written for public consumption, research time might be better spent contacting editors and staff who worked in the 1950's (the time of the Mary Lasswell article) and inquiring what they remember. Lasswell's line certainly makes it sound like the term was institutional by then.
On Very Recent History: The "Slush Pile"
This was interesting, though print research might not be as useful as we'd like. As "slush pile" is primarily professional jargon, spoken between members of a publishing staff rather than written for public consumption, research time might be better spent contacting editors and staff who worked in the 1950's (the time of the Mary Lasswell article) and inquiring what they remember. Lasswell's line certainly makes it sound like the term was institutional by then.