CREATE TABLE table_70791 ("Model" TEXT, "Specification" TEXT, "Build date" TEXT, "Total produced" FLOAT, "Wheel arrangement" TEXT, "Prime mover" TEXT, "Power output" TEXT)

Which power output had a Build date of 1951 1956?
SELECT "Power output" FROM table_70791 WHERE "Build date" = '1951–1956'
SELECT "Power output" FROM "table_70791" WHERE "Build date" = '1951–1956'