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I think you are confusing the compatibility issue. I think you're talking about the OC-E3 off camera flash shoe cord, if so, that is basically an extension cord for the flash, the terminals are exactly the same. The compatiblility issue comes when you're using a camera body that is not the 1DX, 5Dlll or 6D. But is not really a compatibility issue but a fastest sync speed one.
If you're only using TTL flash, the extension cord will work with the 600ex-rt and 500exll.
Thanks for the input.
And yes, I was referring to the OC-E3 cord. I will be shooting it manually and in TTL mode with a 5DIII so it looks like I will be ok.