I took the guitar to a friend's house. He has a great collection of tools. We found a wrench that fit perfectly, and to my surprise it measured as 0.050" (well, 0.0485-0.052 across various facets). I borrowed it. I re-measured my own wrench and got essentially the same values across the flats, but it was still loose in the screw! WTF?, as they say. Then I had a close look at my own wrench (the one supplied with the guitar) with a magnifier and found that the ends had been rounded over so that it no longer gripped. Presumably someone in the shop had done a tuneup too vigorously and damaged the wrench.
So the upshot is that the Allen wrench sizes supplied were correct, i.e.
1 - 1.5mm hex 1 - 4mm hex 1 - 5/64" hex 1 - 7/64" hex 1 - 3/32" hex 1 - 0.050" hex
But the smallest one was damaged on arrival.
