Which owner has a description of Mark 1 pos and is dated 1956?
CREATE TABLE "list_of_great_central_railway_locomotive" (
  "number_name" text,
  "description" text,
  "livery" text,
  "owner_s" text,
  "date" real
);
SELECT "owner_s" FROM "list_of_great_central_railway_locomotive" WHERE "date"=1956 AND "description"='mark 1 pos';