The Cliff Notes version is this: CRK introduced handmade "H" Sebenzas in late 1990. The design looks much like the current 21, with a few subtle differences. They then bought some CNC machinery and starting making production Sebenzas in the same design until the style changed in 1996. From 1993-1994, these production Seb's were marked with a "P". The first couple of hundred were numbered. Starting in 1996, the new style, later called the "Regular" style, stayed in production until discontinued in 2008. In 2000, CRK introduced a concurrent Sebenza line in the original style called the "Classic". Both Classic and Regular styles were produced side-by-side until 2008, when CRK began producing the "21" style, which was very similar to the Classic. Now only the 21 style is available as a regular production knife, with the exception of the Limited Edition Annuals which use Regular style.
Simple, right?