Those 4" black bakelite National type A Velvet Vernier dials come in three distinct versions. Two have the Velvet Vernier logo on the bottom part of the dial, and the other doesn't. I believe the older original version is the one with the logo. Another difference between early and later versions is that the originals have one set screw on the knob. Later, post WW2 ones, have two set screws 90º apart on the shaft.
Of the ones with the logo, there are two varieties: one has very neat, distinct calibration markings/numbers, while the other has rather sloppy ones. At first I thought the ones I had with the sloppy markings had been damaged when Hammy Hambone tried to remove the original white or golden paint with a sharp metal object like a needle (something you commonly see done to a lot of the hamfest finds), but after removing the original flaky paint and repainting the markings on many of these, I concluded that they were that way originally, sloppily cast at the factory. There are also some differences in the way the hub where the dial plate screws to the vernier assembly was cast.
All the ones I have seen without the Velvet Vernier logo have had the better, more distinct markings.
NEVER clean out the old paint with a sharp metallic object. Sharpen a wooden or plastic toothpick, and it helps to use paint stripper. From my experience, paint stripper does not damage the bakelite (but it might not be a bad idea to test your stripper on a tiny spot on the back side of the dial first, just in case; manufacturers reformulate chemical products all the time with no warning). NEVER NEVER EVER clean these dials with a cleanser like Fantastik or 409. It will wash away the glossy skin of the bakelite and expose the filler, which is finely-ground sawdust. Once the filler is exposed, the sheen is lost for ever. Nothing will ever restore it back to original. It is impossible to polish sawdust.
The older original planetary mechanisms look to be brass plated. Later versions are mechanically identical, except they are nickel plated. Some of the nickel plated mechanisms take fine threaded screws while the older ones use coarse threaded screws, #4 IIRC.
To restore the markings after stripping down to bare bakelite, use a white (or gold) lacquer stick, available from Antique Electronic Supply in Tempe, AZ. Rub the stick over the cleaned-out markings, then use a soft cloth to carefully wipe away the paint from the surface of the dial, leaving it embedded in the grooved impressions that form the markings. It may take several attempts to get this right, completely removing all the paint from the surface while leaving the impressions thoroughly filled in. Lacking a lacquer stick, I have successfully used white oil-based artist's paint. Something that helps is to dampen (but not saturate) the cleaning rag with mineral spirits.
On some of these dials, I have found it necessary to place a set of thin flat #4 washers between the dial plate and the vernier mechanism to space the dial far enough away from the panel to keep it from dragging.
National also made a smaller version of the same dial, with the bakelite disc about 3½ inches in diameter, but otherwise identical to the larger dial. Shortly after WWII, the type A dial was replaced by the type AM (type A metal) dial with a nickel plated metal skirt. The newer type AM dial is about the same size as the smaller bakelite type A.