Window properties - how are they implemented?

I'm curious if there is anyone around here that has inside information on the implementation details of window properties (SetProp, GetProp, etc.) in Windows.

SetProp @ MSDN

GetProp @ MSDN

For instance, how are storage and lookups done?

Warning: I looked at this a long time ago, so 1) my memory may be wrong, and 2) it may be out of date, even if I am remembering correctly.

When I looked, I'm pretty sure they were implemented as linked-lists, and performance was quite poor if you had more than a few properties for a window.