Sterling silver bells were introduced in the 1992-1993 time frame as a custom configuration from the Elkhart facility.
from Bach Stradivarius Trumpets & Bach Stradivarius Trombones brochures (2013)
Sterling Plus bells (99.9% pure silver, seamless construction) create a full complement of partials (overtones) in the tonal spectrum, for a focused sound with great projection.
From our images collection of various bell markings, there was both Sterling Silver (earlier) and Sterling Silver Plus (current) bell offerings. The first prototype sterling silver bell Bach’s were made of sheets of sterling silver, while current Sterling Silver bells today are manufactured using an electroforming process resulting in a seamless bell construction. The Sterling Silver Plus silver content is 99.9% pure silver (should probably be called a Silver bell, as Sterling Silver is 92% silver content), while other manufacturers have sterling bells with around 92-93% silver content, and 7% copper content (which is known as Sterling Silver).
If anyone has any additional details, please contact us through the footer of this page.
Source: Luc Decock, Kessler Music (featured image)