Mystery Object

I was digging around in my Box O' Cably Goodness and found a connector that defied my attempts to grok it - it's an RJ-45 (Ethernet) connector that dead-ends into a solid-looking black plastic grip with a hole thru it. At first, I thought it might be a tester for network ports, so I plugged it into my router - no lights, no explosion, and my internet is still up so I figure it's probably not the DoomStick(tm) - but I still have no ide what it actually is.

Any ideas?

On the black plastic body, there's a part number (I think): 750121/A

Here's a pic: