Which power output had a Build date of 1951–1956?
CREATE TABLE "rs_series_b_b" (
  "model" text,
  "specification" text,
  "build_date" text,
  "total_produced" real,
  "wheel_arrangement" text,
  "prime_mover" text,
  "power_output" text
);
SELECT "power_output" FROM "rs_series_b_b" WHERE "build_date"='1951–1956';