That’s the banjo fitting for the oil return from the oil cooler. There seems to be lots of different part numbers depending on year and country of origin. You will need to order 4 parts for the complete assembly; 2 gaskets, 1 union and 1 bolt union.
This is the part no`s for Year 1988-89 Gasket.. 90430-12018 Bolt Union ..90401-12009 Union.. 90430-12018 . A Blanking plug with no pipe is 90341-12012 which is shown in your diagrams. The diagram below shows figure numbers not part numbers.
