All manufacturing marks are absent in an effort to "sanitize" the pistol. This was done in an attempt to make guns more difficult to trace back to its origin if a clandestine operation were to result in the loss/capture of the gun. Meadows pictures a "sanitized" Ruger MK II pistol on page 487 of his book "U.S. Military Automatic Pistols 1945-2012" with a serial number just six digits from this one, "221106". This pistol is identified as a MK I by the lack of a last round hold open and no slide catch. Meadows also mentions that a suppressed version of the MK I was used by the American "tunnel rats" in Vietnam while clearing tunnel systems. One of a few pistols, including the Smith & Wesson Model 59, referred to as the "Hush Puppy" for their roll in silencing enemy guard dogs and guards. According to the book, "In most cases these pistols were re-barreled after being sold to commercial dealers, ostensibly because the original barrels had been threaded for a suppressor." This is believed to be one of those pistols. Collector's marking "JF/illegible" marked in white on the trigger guard.