SCE is shipping the new "PlayStation2" build called "SCPH-15000". Last build was "SCPH-10000". The change affects all units, which are shipping from June 15, 2000.
SCE Public Relations Department said, " The print circuit board gave the change of aiming for the productivity improvement of PS2. The function side and the appearance didn't change..."
Of course we all know that possibly this happened in order to prevent coming PS2 Mod-chips.