Still have mine that I made for myself .... took the back off to have a look at that coupler part.
It's worth noting that the end of the coupler where the threads start can be VERY sharp, I just cut myself and ended up with a quick CA band-aid. It's a tiny little half-moon cut on my left index finger .... ouch!
Also, just where the threads end on the coupler, I noticed a small relief cut in the coupler. I'm assuming that's what is causing the breakage. Mine doesn't seem to be excessively weak. I just tried flexing the pen and over-tightening it a bit, and nothing bad happened. I'll keep trusting it for now.
I currently have several pens that are "mine" .... my first pen, my dragon pen, and most recently my first fountain pen. It's just beautiful in deer antler, but I'll need to invest in a 1/2 inch drill chuck if I want to make more large pens. That was nerve-wracking, drilling it out on my drill press.