Hex Key size question

[two-six]

Hi,
I bought a snazzy Hex Key (with the posh machined aluminium handle) for my new 450L dominator 6 cell to use on the feathering spindle bolts.

I have an ALIGN branded set of these drivers already and the one that fits this bolt has a blue handle and has 2.5 SU'S written on the shaft So I assumed that it is a 2.5 mm one. But it isn't. The one I got doesn't fit.

If I measure the one that fits across the flats it measures 1.9mm's. If I turn it at its widest it measures 2.19mm's. The shaft measures 2.41mm's.

The one I bought measures 2.38mm's across the flats, 2.74 at its widest and has a shaft diameter of 2.92mm's.

It is the same as the other two Align branded drivers that I have already with the pink/purple handles. They have 3.0 SU'S written on them.

I really don't know where you get the sizing of these drivers from.

What is the size of that feathering spindle bolt?
What does SU'S mean?
How do you actually measure the sizing of a Hex Key?
Which measurement is used when you buy them?

Also, so I can still use the handle what would be a good size of a cross-head Philips screw driver head/shaft that I could fit into the handle to use on the small screws you get on the Trex 450 L?

That's all I can think of for now.

Cheers!