Gearbox oil drain ? If not what is it?

I loosened off this (above) thinking it was the drain plug what's it for ?

Drain plug bottom right what is it top right.

 

Probably locates something inside the gearbox.

The 17mm hex is the drain plug

 

Thanks Just found out its something to do with reverse on strip down only moved it a little done back up will leave it alone.

 

Holds the detent plunger into the shaft, dont remove it!!!

 

We had a bloke in last week, with a detent spring and plunger in his hand.

Was absolutely adament he'd undone the right plug to drain the oil out of his motorcycle even though he's lost all gears since.

Couldn't talk any sense into him and gave up trying in the end!