As far as I know, the pencil mechanism that fits the Liberty Pencil is the Schmidt DSM 2005 offered in .07mm.
In the beginning, the Schmidt DSM 2005 was offered as is, as a component of the Liberty pencil, by Constant.
After a while, Constant seemingly modified the plastic cap of this mechanism in order to provide an interchangeable pen/pencil kit or component set. This cap (at the back of the mechanism) is similar to the Parker's cap (see bottom of attached pic) instead of the genuine typical pyramidal Schmidt cap (top of pic).
Also I have to mention that, compared to the Schmidt DSM 2006 and DSM 2007, the DSM 2005 mechanism is slightly different.