Question about Shure SRH840-BK-EFS

BK is the abbreviation for the colour "black" (CL means "clear" at Shure, i.e. transparent (for the in-ears). EFS or E is only an addition to the type designation without any technical meaning.

Shure has meanwhile removed this addition on all websites, as this contributed to confusion (officially the addition is included in the nomenclature - EFS are in any case models for the EU).