What's the difference between Bilstein 5100 Rears vs RCD F4-BE5-D560-T0

I don't post too often but I think I have a interesting question. Back in 2012 I replaced all my 2001 TRD shocks with Billy 5100s from DownSouth Motor Sports. 40K miles later I was looking to replace just the rear Bilstein F4-BE5-D560-T0 (Rt) & Bilstein F4-BE5-D599-T0 (Lt). I happen across shockwarehouse.com entered the Bilstein P/Ns and came back with the following hit which seemed to be a good price.

http://www.shockwarehouse.com/site/...42WD-PRERUNNER-(6LUG),-1.5LIFT-F4-BE5-D560-T0

Being cautious I asked tech support what's the difference between the Bilstein 5100 & RCD. I received the reply, "Bilstein and RCD are the same shocks." and "Yes they fit and yes $62.61 each"

Being an engineer this was not exactly helpful. Reading the RCD description it states "RCD Bilstein shocks use the Bilstein 4600 B6 high pressure (360 psi) monotube design . . ." My understanding is the Bilstein 4600s series are the OEM equipment? So how can RCD use a part of the Bilstein 5100 part number (F4-BE5-D560-T0) in their description?

Sure someone knows the details . . .let me know if I should steer clear of www.shockwarehouse.com too . . . TIA