Colt doesn't issue letters on them because Colt had no control over the quality and production work. Here is an excerpt from the Cap and Ball Revolvers article on 3rd gens.
It wasn't until 1993 that Lou Imperato formed the Colt Blackpowder Arms Company to create what became the 3rd Generation Series. Known as the “Signature” Series, the 3rd Generation Series was under a "licensing" agreement with Colt Manufacturing, but had no connection to Colt at all, except for the use of Colt's licensed name. Under this “licensing” agreement, Imperato at his Iver Johnson factory where the 2nd Generation "F" Series were produced, was responsible for procurement of all parts, which came from Uberti and Armi San Marco, as we have found out, in Italy. Iver Johnson was also responsible for quality control, final inspection, marketing, advertising, sales and distribution of the revolvers. This was a totally different agreement than either Forgett or Imperato had for the “C” Series or the “F” Series and these “3rd Generation” revolvers were never affiliated with Colt Manufacturing and Colt Manufacturing will not recognize them nor letter these revolvers except on very, very rare occasions. Colt will letter those revolvers that have a complete series of manufacturing, final inspection and quality control records that satisfy the Colt requirements for lettered revolvers. These records are available for very, very few 3rd Generation revolvers.
This statement is supported by an article published in the November 1994 issue of "Gun Tests" magazine. It revealed that "Colt's Manufacturing Co., apparently concerned that its customers might be confused by similar names, says it has nothing to do with black powder guns being sold under the 'Authentic Colt Black Powder Signature Series' name." They went on to quote Jeff Crute, Colt's Vice President of Sales and Marketing. He is reported as stating "The guns are manufactured and marketed by John J. Jovino and Co. of New York (owned by Lou Imperato) and bear only the Colt name. The (revolvers) are being produced and sold through a special licensing agreement which was negotiated prior to the current management ( in 1994). CMCI (Colt's Manufacturing Co., Inc.) assumes no responsibility for product quality, workmanship or liability."
