Re: not sure what the DP/N is or whether there is third party selling the same adapter.    I find the answer.  NRX72     [URL]
it is true that many discussion posted already to use third party ATX psu to replace R7 psu as long as the non-Dell psu has two separate 4 pin CPU/EPS connectors, one for the oem board cpu socket, the other for GPU_PWR which is mandatory to make the board POST.  this discussion is about the Dell proprietary 24 to 4 pin adapter.  not sure what the DP/N is or whether there is third party selling the same adapter.   so if I am planning to get a stock 460w psu, I need to get both the psu and the adapter.  most psu sellers do not include the adapter.  although R7 and XPS 8930 share 460w psu, the adapter is not needed for 8930.   the higher power oem psu 850w or 1000w (proprietary modular) provides two cables for cpu and MB GPU_PWR respectively, colored white and black on MB side.  strange that the psu side has three red sockets labelled CPU/MB and R7, R8, R9, only two needed.   not sure if there is an R8 cable included at all.  in this pic R7 and R9 are used.   The proprietary cable are also labelled R7 and R9 respectively so it is user friendly.   note the same 850w/1000w psu are used in Area 51 R2 etc.  The A51 R2 motherboard also has this Dell proprietary 8 pin GPU_PWR socket at lower right corner to be plugged in.  cheapest 850 oem psu here [URL] 
Aurora R7 psu missing proprietary cable