There are literally dozens and dozens, perhaps hundreds, of so called ONE OFF Ruger's in existence, employee specials, both authorized and not authorized -most of which will NOT letter and many other engineering samples, test guns, variations that were authorized and NOT authorized (officially). For example, the tool room prototype engineer would make one-offs based on requests from WBR, WBR Jr., and other higher ups for whatever reasons that can be imagined. Many were made to decide the viability of the design, some were made to determine manufacturing efficiencies or ways to improve (i.e. minimize) manufacturing costs. WBR and WBR Jr. would gift guns to friends, acquaintances as they wanted as the factory were owned and controlled by them. These guns would be made with special features based on the tastes of the individual receiving the gift, the employees were allowed to make guns that were one offs, and because teh employees knew each other and Newport was a very small town, you could buy a standard catalog item at employee, i.e. wholesale price, and get it customized by asking the individual departments to do you a favor by adding a special feature or a child's birthday as the serial number.
There are usually more than one…especially of the official prototypes and many have been sold off to the employees over the years, and are even now, trickling out…the Pilorget is a good example, there are more around than the collectors want to think as many of them are owned by retired employees after the old man decided to sell them off, as they were just taking up space and WBR "wasn't going to pay anyone a royalty!" My understanding is they intentionally under estimated the quantity of prototypes and regular production guns that were actually made, so that the payment to Monsieur Pilorget would be minimized. ( My understanding is the original agreement only allowed for a "few" revolvers to determine the effectiveness of the system prior to any agreements being finalized.)
A good example of this is the custom framed Sefried revolver that Mr. Deerstalker owns, it doesn't say Ruger anywhere on it, is it an official Ruger prototype? will it Letter? Or did Sefried simply use some Ruger factory parts and make them in his own machine shop or unofficially at the Ruger factory???