PART NO.  SPL.PAGE  DESCRIPTION                                                            
2H4685    TC.A.13   Plug for sump -drain -(sump, engine, Twin Cam)
6K431     TC.A.13   Washer for plug -copper -(sump, engine, Twin Cam)

The 2H4685 part is the oil drain plug for the MGA Twin Cam engine (with alloy oil sump). Tricky bit here is an odd thread specification, 3/4-14-ADM (Admiralty Fine Thread). This is a Whitworth thread form with 55-degree included thread angle, but different thread pitch. But it is sealed with a standard 3/4-incn copper crush washer.

At time of this writing, this plug was available from SC Parts Group Ltd in the UK. However, on their web site the thread is called "UNS", which should mean Unified National Special (special being the thread pitch). But, ADM is a 55-degree thread angle, where UNS is a 60-degree thread angle. So either SC Parts is wrong about the thread specification, or I'm wrong about the thread type. (Still checking).