Check out post #7 in this thread...
http://www.914world.com/bbs2/index.php?showtopic=70929I believe you are trying to do what I describe in item #2 in that post. Which is to use a 914 Koni insert in a 911 Koni strut. As you have found out where the insert necks down is too far down and does not match up with where the strut necks down. So it almost, but not quite fits.
For that strut, you need the 911 Koni insert (8641-1039 Sport). On post #11 of that thread I show a similar photo to yours, but you can see that the neck on the 911 Koni insert is much higher, so it fits. The valving between the 914 and 911 in slightly different. Maybe not enough for you to notice. You can always have Koni revalve the 911 insert to the spec of the 914 insert.