That looks like bad metal to me   Maybe when they forged it , it packed all the bad material in one spot  I've seen similar problems on other motor parts like a broken ring with a oval bubble in the material were it broke   I'm no metallurgist, but that break looks like bad material. 
CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax
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