Oil filter head hex nut / bolt , the centre oil filter bolt screws into this with a 3/8" bsf internal thread, with enlarged holes allowing more flow through from filter to engine.

This is not a high selling item unless you are carrying out a rebuild & have lost or damaged it. However in January 2013 we took a DB2 engine to bits & this part had been modified like an MGB with dire consequences - the oil filter was crushed at one end, luckily the car had only been driven 2000 miles in 20+ years & had not damaged the crank.