Momo shift knob?

A while back, hiwind posted this picture of the interior of his car:



The shift knob is from Momo, and this place sells them:
http://www.ltbmotorsports.com/shiftknob.html

But at the top of the page it warns that these do not fit Volvo reverse shifters. What does that mean?

Do we have to modify the shifter in any way to get them to fit? If it's an easy retrofit, I believe this guy will soon be in my possesion...