How to open a thinkpad tablet pen

[charlesd]

I love my X201 tablet, but the pen has only 1 button and I absolutely hate that. I want 2 buttons, like on the Fujitsu.

I found a great 2 buttons pen that work perfectly, the Fujitsu 5000 pen. Now I would like to make its electronics fit inside a X201 pen.

How do you open the pen?? I tried applying various levels of force, the X201 pen just won't come apart where I see the joint. It's OK to destroy the organ donor, but not the organ receiver!!