S-296 was a member of the numerous 'Whiskey' class, Project 613, and was completed in June 1956. The sub was modified to test an experimental closed-cycle engine, which required massive external tanks for chemical storage, as seen in this view. The modification was designated Project KATRAN.

Correct answers were received from: Tom Schoene, Kenji Nozaki, Maurizio Brescia, Rob Locke, James T. Flynn, Jr., Steven Trimble, Craig Howard, Bjoern Rehnfeldt, Bill Vickrey, Edgar Dutkowski, Ricardo Graça Matias, Andrzej Jaskula, Giuliano Ranieri, Voronov Aleksey, Rich Austin, Pieter Cornelissen.