This is the block union fitting that is used with the oil pressure pipe running from the block to the gauge. It is also used with C-AHT9 braided line if your block doesn't already have an oil pressure gauge.
Note the indentation on the end to which the oil pressure line fits. This is as per original specification. Make sure this is what you need if using an after market set up. If not, check OA to see if that is what you need.