CREATE TABLE table_52924 ("Class" TEXT, "Wheels" TEXT, "Date" TEXT, "No. built" FLOAT, "Loco Nos." TEXT)

On what date was the number of locomotives 421-6?
SELECT "Date" FROM table_52924 WHERE "Loco Nos." = '421-6'
SELECT "Date" FROM "table_52924" WHERE "Loco Nos." = '421-6'