I have a 1941 vintage Bankers Special with a one line address, The single line is the " Colt's pt. fa. mfg. co.". My theory is the second line broke or was damaged so what remained of it was removed. With a war on maybe Colt thought it wasn't a high priority to make another as long as the company name was present.
The WWII Detective Specials, some 5,000, consistantly had two line addresses. Mike's theory is interesting. We may never know, that could be it.
In the case of this specific barrel, it's puzzling. 1926 factory 2 inch barreled "pre DSs" had the classic 38 Detective Special stamped on the left side and were blank on the right side. Once the model was completely type classified as the Detective Special it received the two line address stamp on the right side of the barrel. It does not have telltale clues of a barrel replacement. No fixture marks, or worse, gouges. The barrel finish wear is commensurate with the entire finish wear. The front sight looks factory installed and the left side barrel stamping is correctly centered and has the correct font. Yet something is disquieting about this example that makes me wonder if a rebarrel is a possibility even in the face of the revolver not appearing to be rebarreled!
If I have time later I'll pull out TBOCF and see what I can find out.
By then, one of our members will probably post the answer!