For the axes mapping, I think I figured out what the pattern is, why Ry maps to U and Rx to V.
For the primary axes, the order is X, Y, Z. For the secondary/rotational axes, the expected fill order of the axes is Rz (joystick twist handle), Ry, Rx; they inverted the list and in Winmm these latter axes correspond to R, U, V.
The newest patch (coming soon) will have the original enum order {X,Y,Z,Rx,Ry,Rz} and I've written next to the Rx,Ry,Rz comment labels with the corresponding Winmm mapping axes V, U, R. The axes mapping in joyPosEx has been changed to reflect the enum order.
Cheers,
David