[MotoSook]

Well, nothing is conclusive until I have the part in hand (and then maybe), but my first analysis seems to be backwards. The fracture ("stress fracture") likely started in the 6-7 o'clock position in the picture I editied. The area I circled is the final fracture area (shear lip). The two features in the middle look suspiciously like a couple of inclusions that would have added to the weakening of the part. The 6-7 O'clock position (area) looks like there may be radial marks that would indicate it started there. But the crack didn't start in the middle or there would be some coning/cupping effect.

Were the camshafts cased then machined? Or were they forged..did they forge them back then?

If the casting had an inclusing that was machining into, it is likely that this part was destined to fail at that spot. The crack doesn't look a fatigue failure, so that part was probably bummed from the start. It just failed....

You ARE luck it happened during assembly.