Marcel Proust, George D. Painter: "But the most important of these unprinted articles was an essay on Dostoevsky, which Proust proposed to Rivière in September 1921, and then abandoned, fearing it would interrupt his novel for too long."
Presumably the same as the Dostoevsky article printed in Nouveaux mélanges and Contre Sainte-Beuve.
Although:
Marcel Proust, A Life, William C. Carter: "When Rivière requested an essay for Dostoyevsky's centennial Proust refused to be distracted from revising his manuscript. Although he admired 'the great Russian passionately,' he knew his works 'imperfectly'."