Ahh.... I see.
My best guess is that it's bore-axis, muzzle down, perpendicular to the ground, or belt, when standing is "12:00", unless someone tells me different.
So breech-end of the barrel axis is the pointer on the "hour hand".
Pistols and revolvers all have so many diffefrent shapes, grip angles, and reciever configurations, angles, and sizes, etc. almost all else is meaningless, but they all have a bore, and they're all (hopefully!) straight.
That's my best guess if you want a fixed standard that can be used to describe holster cant, for any kind of pistol or revolver.
Otherwise talking about "o'clock" for holster cant is just an approximation to be used in discussion, because in all other respects, all guns are different.