What is the highest production code of the episodes having a US viewership of exactly 2.3?
CREATE TABLE "table1_28787871_3" (
  "no_in_series" real,
  "no_in_season" real,
  "title" text,
  "original_air_date" text,
  "production_code" real,
  "u_s_viewers_millions" text
);
SELECT MAX("production_code") FROM "table1_28787871_3" WHERE "u_s_viewers_millions"='2.3';