I believe the thin u is more rounded and thicker whereas the thin flat is possibly similar to an ibanez if thats what my m-202 had its not that great neck for me to play on....
I think the ex thin flat is like 19-21mm or thinner adn the thin u is 20-23 or something like that.
Shane would have more of an exact idea considering he's played most of ESP's neck profiles....
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