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

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'