I may not be an engineer, but I know a stress riser (a starting/intial point for failure) when I see one and I see two in these pics. Both resultant cracks are identified by Christian in the two circles he included on the pics.
What I think caused the cracks is the inward tapered milling on the bottom of this (I assume) short plate. When the taper crosses the screw hole(s), a thin section of metal is formed and a stress riser is created and that's where the crack starts. For you aviation history buffs, this sort of like how the deHavilland Comet taught us that square windows are bad for pressurized fuselages.
I checked my OS2 long plates from a year or two before yours and there are no flaws like those on your short plates that would cause stress risers. My guess is it's either a design flaw in the short plates or a manufacturing flaw in your (and possibly other people's) plates. I can't guess what effect, if any, the Bomber heel receiver contributed to the failure. I'm leaning towards none, but the lighting does make it look like the contact area between the steel receiver and the short plate resulted in a disfigurement of the softer aluminum of the short plate. I just don't think that alone would cause this. I think it's clearly the stress risers.
Other notes:
Corey, I agree, if there's a pair of holes for the heel receiver, you should be able to use them. Otherwise, they shouldn't be there.
Crimson, I didn't catch what his full setup is, but, if he's got the D3 elastomers, he doesn't need the o-ring under the king pin.
Bottom Line:
Every CATEK user should check their plates for this flaw.