So. I got my beautifully made M14×1.5mm delrin blanking plug and now I know that the way you measure blanking plug sizes isn't the way I thought.
I've got an M14×1.5mm blanking plug for sale if anybody's interested.
The bicycle parts I linked to are probably the cheapest way of making certain.
https://www.fischerconnectors.com/si...6_june17_0.pdf page B1-10