Which Built at has a Date of 1946, and a 1947 Nos of 3556/7?
CREATE TABLE "construction" (
  "1946_nos" text,
  "1947_nos" text,
  "date" real,
  "built_at" text,
  "lms_nos" text,
  "br_nos" text
);
SELECT "built_at" FROM "construction" WHERE "date"=1946 AND "1947_nos"='3556/7';