Further more, at GM/Holden we used to call Christmas Tree clips "Fir" tree clips...I guess to avoid the use of the word Christmas LOL! :)
Fir tree:
Proper "scrivet" where the centre pin can be unscrewed with a Phillips screwdriver:
This is the one I've seen on the Volvos (for example C30 under-dash sound panel). Put it in the hole then push the pin. To remove you push the head of the pin farther in then remove fastener and re-set pin position to re-install:
This is starting to remind me of an episode of "Fun with Flags" with Dr Sheldon Cooper...