The P was to indicate that Sebenzas were now Production, instead of Handmade. The first few hundred had numbers. P1, P2, etc. After that, they just had the P for a while without the number. Shortly after that, CRK dropped the P altogether, but the design of the knife didn't change until 1996 when the design we call the Regular was released. All Originals were made with ATS-34 steel.
Thanks Long_Wolfe! So based on your explanation, may I assume that Original Sebenza was actually the P series without the P mark, and it's not a real series but more like a title?