This is a huge undertaking....but it needs to be addressed. I think the best angle of attack is to divide this group into banjos produced in the 1920's, which had a tube and plate flange, and those produced in the 1930's, which used a cast zinc alloy flange. We are going to be dealing for the most part with prewar Mastertone banjos, since those are the most often copied. There isn't enough incentive to reproduce most of the later models, if the intent is to defraud. Even those banjos have been copied, though, so I may want to pass a few notes along on those in yet another section.
The best way to spot a fake, is to know what to look for in an original...
1920's Mastertone models.......
Typically, Mastertone banjos produced in the 1920's have a 2 piece flange, thick rim (measuring a full 7/8" thick below the flange). Styles 6 and up continued to use the 2 piece flange well into 1931.
The fiddle shaped headstock was used on all Mastertone models during the years 1925 to 1929 on styles 3, 4, 5, 6*, Granada, Florentine and Bella Voce(The last 4 mentioned were not marked Mastertone).
All Mastertone models used a Brazilian Rosewood fingerboard that was scarcely more than 1/8" thick. The style 6 was nearly always ebony, and Bella Voces and Florentines can exhibit either wood, although Rosewood seems to be more prevalent.
Resonators on style 3 banjos produced in 1925 and 1926 are bound only on the back, having no binding on the edge nearest the flange. The resonator on the 1925 model was held in place with 4- 6x32 thread bolts having a 1/4" hex head. This gave way to the familiar thumbscrews in 1926.
1925 through 1927 Mastertone banjos used Grover pegs with screws into the peghead. There are 2 styles of these pegs, one having opposed tabs which protrude from the housing, and another style which had a more formidable cast housing, roughly diamond shaped with 2 mounting screws in each. In 1928 the more familiar Grover "pancake" tuners were employed. The Florentine and Bella Voce models used small shaft planet pegs, usually with pearl buttons.
Serial and lot Nos. appear in 4 places in a Mastertone banjo manufactured prior to WWII. The Number will appear in the resonator, along the edge near the cutout for the neck(these appear to have been painted), and in white grease pencil across the interior of the resonator. They will also appear stamped into the rim's interior, and on the heel end of the neck where it joins to the rim, usually in pencil, but sometimes stamped as well. This one is only visible with the neck removed.
Heel fitting on these banjos was achieved with chisels rather than by sanding, so if it looks nice and neat and machine ground, it's probably not real.
There are several areas to pay attention to, and I'll list some of the things that I've seen most often.
Fingerboards...should be rosewood for the most part, and around 1/8" thick. Alot of repro Gibson necks have been built on Neck blanks, or with parts from Stewart MacDonald, or other suppliers of banjo parts. Alot of these will have full 3/16" fingerboards.
Truss Rods...will have a 5/16" hex head nut of brass, with finish on them, these areas were not masked during the finishing process, which is something many modern builders do. The presence of a 1/4" fitting certainly says...."reproduction"
Inlays...will be silvery, with nearly the appearance of aluminum. The presence of alot of color, or figure suggests a modern reproduction. Similarly, beware of inlays that have a bluish cast. These have been set (usually in ebony) with aniline dyed epoxy, and show the color through the pearl.
Binding...While some of the scraping may give the appearance at first, Gibson didn't use grained ivoroid binding. The material was a cream color.
Scale length...should be 26 3/8". There have been different scale lengths available, and marketed as Gibson's prewar scale. Assuming that they didn't use a different scale length for plectrums, the correct scale length for a prewar Mastertone is 26 3/8"
Headstocks...were nearly always painted black. I have seen some dyed pearwood headstock veneers on Ball bearing models from 1926. but by the time you get into the 30"s most were painted. I have never seen a prewar Gibson Banjo with an ebony headstock veneer.
Finish...should show the same type of checking on the neck as is exhibited on the resonator. Beware of finishes that look very thick and "glasslike" with long straight checking.....this is nitrocellulose lacquer, and was not in use. Prewar finishes were much more delicate, and the checking is very small, and stacked in straight lines.....vaguely resembling a nice stone wall.
The industry of vintage instrument dealing will benefit from folks' having access to this information. The fraud has gone unchecked for too long, and too many people have already been ripped off.
DR Z's article on EL84 tubes
CyberMonk's guitar tone
and Mr. Keen's Tube Faq