Found this info in the book "Standard Catalog of Smith & Wesson".
The Tunnel Gun : a highly modified N (29-2) frame with a 1.3" smooth bore barrel, blue finish, chambered for a "very" special .500" noiseless shot shell. Manufactured for armed forces use in Southeast Asia by the A.A.I. Corp. Also called the Tunnel Weapon or "QPSR" (Quiet Special Purpose Revolver). On display at The Military Ordinance Museum, Aberdeen MD. Apr0x. 17 manufactured, 1969.
A noiseless shot shell? Must have been very special indeed.