Referencing the other thread. v7.71 enables the new bitmasks but the /help lists only those up through 16.

1 Left mouse button
2 Either Ctrl key
4 Either Shift key
8 Either Alt key
16 Right mouse button
32 Middle mouse button
64 Right Shift key
128 Right Ctrl key
256 Right Alt key
512 CapsLock
1024 ScrlLock
2048 NumLock
4096 Left Shift
8192 Left Ctrl
16384 Left Alt

I'm taking it on faith that 32 is the mouse's middle button, because I don't have one. A middle button, not a mouse.