The Fingernail Piercing Tube is a device designed to puncture through the fingernail causing intense pain and lasting damage.
Tobaku Mokushiroku Kaiji
Ichijō uses the device on each finger on the right hand of Kaiji. The damage inflicted remains throughout the whole arc as a reminder to Kaiji and the trouble he had went through to reach that point.
Tobaku Datenroku Kaiji: 24 Oku-hen
The device later makes a reappearance this time it is used by Endou to extract information from Sakazaki about Kaijis whereabouts as Endou was assigned by Teiai to find and track down Kaiji and his group and retrieve the ¥2.4 billion they allegedly stole from gambling with Kazuya in their gamble in One Poker.
Unlike Kaiji, only one of Sakazakis fingers is pierced as they receive news on a new lead to Kaijis whereabouts before Endou has the opportunity to pierce another.