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I think there was 2 versions. The original (current codex) and the reprint with some typos fixed. If your description for flayed ones says it doesn't work on Fearless units, then that's the current. My book doesn't say that anywhere in the description and makes me believe I have the original (which should be right as I bought it close to the release).
"There is only do, or do not. There is no try." - Yoda
I believe another clarification is the addition of Living Metal negating things like powerfists and other strength modifications in the newer codex. This is something my codex doesn't have for sure but I've been told it ended up in the codex by newer players.
As the codex is same except for those few little clarifications it should not matter which one you have. However, as our codex is originally written for 3rd ed. and rules have changed a "bit" from those days you should download Necron 5th ed FAQ from GW site and have it available as you play.
Revenant Moon Necrons: (W/L/D) 0/1/0 (6th edition: 13/2/2)(5th edition: 14/6/4)(3rd edition Codex: 16/4/7)
XIX Legion: (W/L/D) 0/0/0 (5th edition: 14/12/2)(5th edition Codex: 4/3/0)
Black Legion: (W/L/D) 1/0/0 (6th edition: 2/3/0)
Something Wicked... Reikwald Beastmen: (W/L/D) 10/4/1