Although it is a small member,
Knurled in the tip threaded part,
Hexagonal wrench with 12 mm diagonal can be tightly screwed in,
As it is made of brass that has a reasonable strength / suitability as a water supply piping material and excellent cost-effectiveness,
I have no complaints at all.
There are variations at lengths of about 5 mm intervals, which is very convenient.
Although more expensive cheap items may come out,
Currently, I do not mind using Mr. San-Ei's polygonal hole sarbo.