Bakumatsu Rock: Mystery! Onsen Kaijiken ze yo!!

幕末Rock 湯煙推理劇[ミステリー]! 温泉怪事件ぜよ!!

Unaired OVA bundled with the game Bakumatsu Rock: Ultra Soul. An incident has led to someone's death at Teradaya, which the gang went to for a day trip. A "69" on the victim's leg will be the key to solving the mystery. (Source: MAL News)