Table: new_york_undercover_episodes_2
Columns: series
season
title
directed_by
original_air_date
production_code

Question: What is the production code of the episode before season 8, with a series number less than 31, and aired on September 21, 1995?
SQL: SELECT "production_code" FROM new_york_undercover_episodes_2 WHERE "season" < 8 AND "series" < 31 AND "original_air_date" = 'september 21, 1995'