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The tube-and-plate models required an added lip to support the tube and did not require machining down of the rim, leaving the tube and plate rims to be a full 3/4″ thick.
Flanges: The tube was the first non-shoe system used by Gibson to secure the tension hooks (that tighten the head).
The letters were followed by a number indicating the grade or quality of the instrument: -00 (double zero) was bottom of the line (although there was a short-lived “Jr.” model which was the least expensive); -0 was next; -1 was slightly better, and usually meant nickel plating and plain-colored finish; -11 (double 1) was a secondary inexpensive version; -2 followed with fancier inlays and extra binding; -3 was fancier; -4 fancier yet; and -5 (for a brief period) was the fanciest model boasting gold plating, choice curly maple, and elaborate inlay designs.
Coordinator rods: One of Gibson’s developments for attaching the neck to the rim was the “coordinator rod” system; two brass rods that attached to two screws in the heel of the banjo neck.
It has been thought that the spring loaded “ball bearing” tone chamber was designed to counter the effects of weather changes on the skin heads.
This is not true, especially since a vertical change in the head’s position would cause improper playing.
To improve the cosmetic appearance, a poorly glued three-ply rim would be placed on a lathe and a “cut” made into the glue joints.
Then, thin filler strips would be glued into the cuts and then machined flush, resulting in a multiply appearance, while still basically a three-ply construction.