I've got three Asian made fender style guitars, (Ibanez and G&L) and one USA 1994 Peavey. The saddle height screws vary annoyingly in hex hole size, even within the same guitar. I have a hex key that measures 1.50 mm and it fits good in some and is too loose in others on the same guitar. By too loose I mean it still works but just barely and if it was looser there would be risk of it stripping the hole. I have a hex key that is 1.54 and it is too big for some of the holes but it fits perfectly snug for some others on the same guitar. I think my MIJ Ibanez is the most consistant and all of its screws fit the 1.54mm key best. It is the highest quality guitar of the bunch and I believe all of its screws are stainless steele. It is the most likely to be metric but why would its holes be 1.54mm instead of the standard 1.5mm? Is the standard adhered to for hex keys but the holes are made bigger within a tolerance for safety margin so that most out of tolerance too big hex keys will fit? If they were SAE 1/16" that would be 1.58mm and I don't think that would fit in any of my hex holes so they must all be metric but why the wide variance? Is my micrometer inaccurate? It is consistant. I was about to buy a high quality 1.5mm hex key but maybe I should buy better screws.