Hi @SonicWallBill welcome to this free user-to-user Alienware laptop discussion forum. 

Support for Alienware X51 R2 Documentation has no Dell Part Numbers (DP/N). However, the $30USD DP/N WKGR1, 0WKGR1 is acknowledged online Here as Dell OEM Alienware X51 R2 / R3 Desktop CPU Heatsink Fan Assembly with Shroud is DP/N WKGR1.  

This was acknowledged on eBay as New Dell Alienware X51 Heatsink WKGR1 with 4-Pin Fan 7C20C + Cover DVT3V RR3J2. 

Have you tried this web page at Hard Disk Direct? The part number you listed is for the entire assembly. 

Thanks for the response.  Both of those are out of stock, which is the same everyplace.

That is exactly what I need though ... the bottom part with the screws.  Shoot, I'll take the whole thing just to get that part I need!

Hi @SonicWallBill thank you for sharing that bottom part with the screws is required. 

Used search New Dell Alienware X51 Heatsink WKGR1 with 4-Pin Fan 7C20C + Cover DVT3V RR3J2. at DuckDuckGo.  

Dell Alienware X51 R2 Heatsink 7C20C WKGR1 & Fan 0.75A not 0.6A Version is available, example Dell Alienware X51 R2 Heatsink 7C20C WKGR1 & Fan 0.75A not 0.6A Version US Stock_PRODUCTS_server parts deals for server parts picker There may be discounted offers online. 

Hi @SonicWallBill here are some online reviews that provide additional information about the Alienware X51 R2 product build with pictures and video. 

Alienware X51 Small Form Factor Gaming PC Review - YouTube 

Alienware X51 R2 Small Form Factor Game PC, Haswell-Infused | HotHardware 

Alienware X51 R2 review | Expert Reviews 

Crucial System Advisor database Alienware Alienware X51 R3 | Memory RAM & SSD Upgrades | Crucial.com.