1 x 2 brick with square hole on top identification
Well, between my comment and zovits "seconding" the idea, I think it's safe to assume we've established what this part "was".  I did a lot of searching earlier online, looking through people's collections of non-production and prototype rare parts and couldn't find this "version" of the part.  So, whatever the story of how it came to be, I would certainly consider this to be a rare keepsake.  Here is a photo of the inside of these bricks with the square insert from the "finger" easily identifiable.  

  

I couldn't bring myself to attempt a disassembly since I apparently don't have many of these and they aren't the more common color of Black and Dark Gray.  Perhaps Rin Rio-Oki will happen upon this question and take the initiative of dissecting one?  Fingers crossed ;)