Oil pressure sending unit question.

Hello all,

I am building a 1969 383 and went to prime you'll system when I noticed two fountains shoot our from the rear of my block. I have recently learned that one of those holes are meant for an oil sending unit, however I am writing to you all for help on what is intended for the other hole on the back next to the oil sending unit?


Thanks to you all and your very much appreciated help!

Mike