The ship SSBergensfjord built before 1973 has what tonnage?
CREATE TABLE "passenger_ships" (
  "ship" text,
  "built" real,
  "in_service_for_nal" text,
  "type" text,
  "tonnage" text
);
SELECT "tonnage" FROM "passenger_ships" WHERE "built"<1973 AND "ship"='ssbergensfjord';