Plug near oil filter (Disco II)

I'm getting ready to do my first DIY oil change on my '03 Disco II (once my filters and oil plug gaskets get here) and I had a question.
I've found my oil filter up under a shield but right next to it there is some sort of drain plug.  I know it's not the oil drain plug, but what is it?  If you're under the truck it is to the left and slightly above the filter. 

Thanks for the feedback.

-Ugly

Sounds like the outlet where you can hook in a mechanical pil pressure guage.
What kind of oil filters did you order? Make sure you fill he oil filter before you reinstall it.
How many miles on Your truck?
Mike

Stock filters from Atlantic Brit.  $40K miles, but out of warranty on time.