rod mix-up? question

I have a set of supposedly 340 rods,I got with my 340 block and x heads unassembled.So asI'm cleaning the rods up I notice the 7 are 645 bushed and 1 is a 496 bushed rod.So I assumed I had a set thrown together but then I noticed the stamp is the same as on all the rods and the rods themselves look almost exactly the same other than the 496 has a rounded belly in the web and the 645 is more squared off,more like an I beam.
So my question is,could or would the factory let one slip by or is there really no difference in the rods.Also it apears they have been balanced so could the PO put this together and it all balanced out?I'm in the process of hunting down a scale.
This is a side by side to show the similarities