Rubber blackjack

The Rubber blackjack is an item used to knock out the advanced pickpocketing trainers or coshing volunteers once the player has lured them. It is similar to any blackjack, except that it can only be used for training within the Thieves' Guild. It can be bought from Dodgy Derek in the Thieves' Guild for one coin after the blackjack part in The Feud and two capers have been completed. The rubber blackjack knocks NPCs out for the same amount of time as a regular blackjack aswell. If you use a rubber blackjack, there's a higher chance that it will fail and give you a massive boner, leaving you to be attacked by your victim.

Trivia

 * Although the blackjack is rubber it can knock out a large person, and does so for even longer than the highest quality wooden blackjack. It is possible that the villagers, bandits, and menaphite thugs (the regular blackjacks are used on) are more resistant to blackjacking, or that the volunteers are simply pretending to be knocked out.