I was testing out the other trackpoints that came with my Thinkpad T400 the other day, and when I took one of them off, the black piece that fits the track point cap onto the little nub in keyboard came with it. When I tried to pry it loose it went flying off, bounced twice, and like all small, critical pieces do when they hit the floor, disappeared.
I haven't been able to find it, and without it the caps won't fit. I've looked around but it seems nobody offers a replacement. They just have the caps. In the end, luck was with me and I found the part on the floor.
But, what is this small black plastic adapter part and is there a P/N to order it?
The trackpoint is actually a small separate circuit and controller stick that is mounted to the bottom of the keyboard and extends upward thought a round cut out hole. The circuit board is protected by a bit of aluminum shielding.
Many current versions do include a small rectangular adapter that snaps onto the smaller square stem of the trackpoint mechanism. This can be seen when the rubber cap is removed.
This small adapter is not intended to be removed along with the rubber cap. If it comes loose, it can be pressed gently back into place and then the new rubber trackpoint cap / cover installed onto it.
Lenovo does not have an orderable part number for this small plastic part as it is not intended to be removed / replaced. If lost, it would need to be scavanged from other keyboard or the keyboard replaced.