I've finally got round to doing something about this. As usual Steptoe

was right about the drain plug. I have the type that doesn't need a sealing washer.
I have the type on the right. However the sealing surface on my gearbox is definitely a 'bit chewed', where a previous owner over-tightened the drain plug. The bike had a sealing washer fitted to the (washerless type)plug when I got it, which I can only assume was an effort to make up for the damaged sealing surface.
I decided to order the proper type of plug to use a sealing washer (type on the left). The parts fiche suggested these part numbers were correct
drain plug 23107651450
washer A14X18-AL
But what arrived looked like the plug on the left (ordinary hex head not socket cap), but with no taper to centre a sealing washer like the plug on the right!
Have I got the part numbers correct? Does anyone have a spare washer type plug, they want to part with? Or should I just buy a generic M14x1.5 drain plug designed for a sealing washer and use that?
I thought this was going to be easy