Extra plug in back of block - Extra oil pressure tap?

[sho thing]

There is an apparently unused port in the back of the block with a brass plug in it, down and to the right of the oil pressure sender. Bottom right in this picture:

What is it? I was hoping that it's another oil pressure tap, so that an auxiliary dash mounted oil pressure gauge could be installed without parting the computer from its stock oil pressure sender and causing the dash light to be on all the time. BTW, is the stock sender just a switch, or an actual transducer?

 

[yamahaSHO]

That's the rear coolant drain plug.

If you want to put an auxilary guage on and keep your stock dummy light... Put a tee connection on it an connect both the stock and aftermarket sender... I'll take a pic of mine tomorrow.

I don't know if stock is a on/off or variable.. but it's not a transducer. It does not actually turn mechanical energy into electrical.