Scratching my head
Now I'm really curious what's going on.
I got the unthreaded CO2 cartridge in the UltraFlate, but I had to push it. Wasn't able to get the threads of the UltraFlate together. It was as though the cartridge were too long. Checked it to be sure, and it is a 16g cartridge.
Another rider is a retired engineer, so I defer to him on things mechanical. He tried as well, and he concluded the same thing. Cartridge is a bit too long.
Maybe Mercury is in retrograde for me. I've had 4 flats and a pinch flat tube explosion in the last 5 weeks.