X2^^ Agree with John, however I'd suggest that you take the existing plug out and take it with you when you buy a replacement. As John said, it is not standard and the size &/or the thread may have been changed. Also 3/8 BSP thread is about 17mm diameter.
The bolt (nut) you referred to, which is higher than the level plug is to hold the iintermediate shaft (which you can see the end of next to the nut) in position.

