Here is a picture. The prop adapter slides all the way down the P42 shaft until it 'bottoms out' on the C-clip at the gear box. When the collet fits over the prop shaft, it protrudes beyond the prop adapter - it too hits the c-clip. If you remove the prop adapter slightly from the gear box, the slots cut in the prop shaft extend beyond the tip of the P42 shaft. The collet on this prop adapter is very thin, so it does not support the prop shaft for the length of the slots like most others do.

So, the tip of the motor shaft is not 'buried' in solid metal, it is only surrounded by 4 tongues of 1mm aluminum. I don't see what would keep the aluminum from bending or stretching at the end of the motor shaft with any side load. This thing is supposed to be able to swing 21-22" propellers. I don't trust it. The slots should be at most half the length of what they are now, such that they are fully surrounded and supported by the clamped-on collet.

This is disappointing, mostly because it's coming from the maker of (in my opinion) the best motors there are.

The $6 Great Planes adapters aren't bad! They have a 3/8" shaft though (slightly less than 10mm but fairly robust), and require drilling and tapping for the spinner. They seem to be the best thing out there (barring the pricey Tru Turn adapter which I have not seen in person yet).
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXMSG7