What was the production number of the episode filmed in aug/sept 1968?
CREATE TABLE "table1_1971734_1" (
  "episode_num" real,
  "prod_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "originalairdate" text,
  "filmed" text
);
SELECT "prod_num" FROM "table1_1971734_1" WHERE "filmed"='Aug/Sept 1968';