Groper Train: Search for the Black Pearl (1984)

Directed by Yojiro Takita

The world's largest black pearl has vanished without a trace, and it's up to Detective Ippei to track it down. His only clue is the mysterious words of a dying man, who managed to mutter "Pussy Print!" before slipping away forever. Teaming with the man's younger widow Matsuko, Detective Ippei boards a local train and begins taking prints of the female passenger's vulvas in hopes of matching one to the print that Matsuka's husband had in his possession at the time of his death.