Whether the milled out areas of the receiver and frame ( and the notch cut into the base of the frame), enhance the visuals of the Colt TSMG is, I guess, subjective. And even this part has been seen in several versions with one attributed to Colt manufacture and others to unknown manufacture. The rocker is the only new part that was needed for the conversion. While the original four (4) internal frame parts are what makes the 1927 "different" from the 1921/28, three (3) of those parts are just modified original 1921/28 internal frame parts. I currently have the ability to turn 2 additional 1928 Navy’s in to scarce guns, but I do not have the ability to mill out and stamp receivers and lowers, nor do I have the ability to manufacture the internal parts required to make the 1927 correct. Now, in my humble opinion the original 1927 with the correct internals and the milled receiver and lower is much harder to locate than furniture, (which the wood is) as this is the only factor that would make the 1928 Navy with same scarce. Now, I also have a 1927A(5066) that has the correct semi-auto internals as well as the in the white milled out receiver and lower. I also have two complete sets of Colt Thompson wood for the Colt made 1928 Navy with the above (short fore grip and off-set swivels). The fore grip is about ¼ inch shorter than the military version found on AO/Savage mfg. I currently have in my collection a 1928 Navy with the correct, short fore grip and offset sling swivels (8066).
I would disagree that the 1928 Navy with the correct horizontal fore grip and offset sling swivels are scarcer, or more desirable than a correct 1927.