Unidentified drain hole under 8V92

[Ryan Wright]

Found something I hadn't noticed before while working under the coach
today. Just in front of the 8V92's oil pan, there is another
oil-pan-like object with a threaded hole much like the oil pan drain
plug. However, this hole doesn't have a plug screwed into it - it's
just open to the outside world. I took pictures:

http://www.ryanwright.com/tmp/bus1.jpg
http://www.ryanwright.com/tmp/bus1close.jpg

Anyone know what this is? Hopefully it's supposed to be unplugged?

Thank you!

-Ryan
'86 PT-40 8V92