Allison parts catalog PC3062EN, Service Information Letter (SIL) section SIL 41-45 tells all about the C2 piston/sealing ring type. Early 01-02s with C2 piston #29538342 (with .060" bleed hole in the bottom) used butt joint sealing rings #29538212. Later 03+ used piston #29539653 (bleed hole ELIMINATED) used double lap joint sealing rings #2953894. In the Description of Change column it states: "Improves consistency of C2 fill volumes".
Hope this helps
My bleed hole is a .040(double checked it via 2 different bits and a micrometer), and i tried it both ways with no change in the 3-4 shift.