What is this plug on top of the motor?

[jamesf]

Hi all, simple question that I can't seem to find the answer to, probably because I don't know what to search. What is the plug on the engine case behind the cylinder head underneath the seat? Photo of the part in question (Circled on this image: http://imgur.com/a/NkkzB)
Reason I'm asking is because I saw the port and thought "Oh, I bet that would be a good way to bore scope my trans case and look around" ... all I found was a little pool of oil and a small spring. Now I want to make sure I didn't possibly misplace a little piece in there or something.
And yes I know I shouldn't go taking things apart if I don't know what they are, curiosity killed the cat and all.

[sledge]

Its the detent spring, part of the transmission, basically it stops it jumping out of gear. Its vital, make sure it goes back.

[jamesf]

Thank you.