I've used this 'K&M 216 thread adapter', to mount the 'K&M 211/1 boom arm' which is 3/8 thread, to an 5/8 threaded mic stand.
It does it's job well for over 2 years now.
The adapter is strong & robust, and makes a flush tight fit between the two. The internal threads/grooves holds well and doesn't slip.
The outside texture makes it easy to grip, screw on & tighten by hand.
The un-screwing after use might be a bit rougher for the more delicate finger tipped among us.
One might opt to use pliers to un-screw for the sake of comfort.
But over-tightening is far from necessary.
Too bad it's not available in black.
Since most mic stands & boom arms are black, it would've made this adapter less visible to the eye.
Something the R&D at K&M should consider ;)
Hopefully with reading this review ;D
Cheers