Just to clarify the names. The revolver that was adopted by the army in 1873 was called by them the Model 1873. It was .45 caliber. Colt made the first guns for the Army on contract, then began to produce the identical gun (except for the lack of the "U.S." marking) for the commercial market. Today, all Colt revolvers of that model, regardless of caliber and whether they were part of the army contract, are commonly called "Single Action Army" revolvers. So the "Colt Frontier Six Shooter" in single action is one variation of the Single Action Army. (The .44-40 chambering of the Model 1878 double action revolver is also marked "Colt Frontier Six Shooter" in the roll-marked format, not the etched panel.)
Also in 1873, Winchester introduced its Model 1873 rifle, in .44 Winchester Center Fire (.44-40) caliber. About 1878, Colt responded to demand and chambered its SAA for .44-40, allowing those who wanted to own both a rifle and a revolver in the same caliber.*
Colt, apparently for marketing purposes, chose to mark the first of those guns in .44 WCF (.44-40) "Colt Frontier Six Shooter" in an etched panel on the left side of the barrel. Later that marking was changed to a roll-marked legend.
Many people who read (and some who write) books on the Single Action Army see the part about the etched panel and stop there. But ALL First Generation (1893-1940) SAA revolvers in .44-40 were marked Colt Frontier Six Shooter. Here is what Wilson says, "COLT FRONTIER SIX SHOOTER was the standard marking for the .44-40 caliber revolvers, and in its initial form appeared etched."
Note two things - the words "in its initial form", indicating there was another form, and the term "appeared etched". But what was etched was not the lettering, but the surrounding area, leaving the lettering raised. That is why I used the term "etched panel" rather than "etched lettering." There is nothing that "appeared etched" about the lettering; the surrounding panel is clearly etched. Re-reading that sentence, I really have to wonder if Wilson ever actually saw one of those early revolvers.
Now, an interesting point. Colt started marking the .44-40 revolvers with the "etched panel" c. 1878. At almost the same time, they began making the Model 1878 double action, with .44-40 guns also marked with the "Frontier" legend but ALWAYS rollmarked. AFAIK, there are NO Model 1878's with etched panels.
So, it is one of those things that is impossible to prove, but it is reasonable to suppose that with a new model coming out in .44-40, Colt decided that the etched marking was too costly and time consuming and went to the roll mark on ALL its .44-40 barrels. (The barrels are identical and interchangeable.) That would explain why the etched marking on the SAA lasted such a short time, and is so rare.
But without that information, it is easy for the novice to be misled into believing that the "Frontier" mark is rare and exotic, rather than appearing on fully 1/5 of all SAA first generation revolvers (64,489 out of 310,386). Only the "etched panel" marking is really rare and worth a large premium.
*Rifle makers generally shied away from chambering the .45 Colt because of extraction problems caused by the small rim; the Colt revolvers had no problem because they used rod ejectors.
Jim