The LSC has just ratified the LDraw.org Parts numbering scheme for parts with unknown numbers. [4]
It should allow to add parts to the library where the part number is not known. As the systematic that is used up to now has reached its limits this change is necessary.
Please read also the FAQ for LDraw Part Numbers [5]
Michael Heidemann
On behalf of the LDraw.org Standards Committee (LSC)