Thank you for taking the time to reply, let me clarify things a little.
When I first tried the .reg file from the main article on this thread it didn't add anything to my 'Computer' right click context menu, not a single new entry.
Then I tried playing around with the registry entries I mentioned and by copying the 2 values from the pre-existing 'Manage' context menu entry (the one that has the blue and yellow UAC style shield icon next to it). I was first able to get a second 'Manage' entry to appear in my 'Computer' context menu, this one had the icon depicted showing the little toolbox in front of a computer though, when I clicked this new 'Manage' entry it opened the Device Manager. The other one opened a window with a title of 'Computer Management'.
I was hoping it could get it to actually show as 'Device Manager' in my 'Computer' right click context menu so I tried removing the ",-400" from the copied values. This time after restarting it successfully showed up as 'Device Manager' and even had the correct icon still too. And this new 'Device Manager' entry functioned as the original .reg file was supposed to (even though it didn't for me until I started playing around with regestry entries within the key that they .reg file creates only, no other keys were modified by me).
Unfortunately around this time I noticed that my Hard Drive icons were 'missing' when I opened 'Computer'. The image below is clipped from a screen shot and depicts the drives listed in 'Computer', as you can see the icons for drive C:/D:/K: are missing their 'normal' icons.
The new test file you shared worked right off the bat to add the 'Device Manager' entry but even though I had deleted the changes I had made in the registry before using it I still have messed up drive icons.