If you are using a mechanical oil pressure gauge in your car then this simple special brass adapter is sure to be needed. It will either screw into the block directly or into a "T" piece to accept the pressure line fitting. This is the fitting at the block when using the braided hose C-AHT9.
Note on the left side of the photo the type of "male" fitting used. This works with some after market gauges. For the original Classic Austin Mini gauge fitting, see 2A269. For the small washer by itself, use 2K4936B.