Thanks man :-D
I feel a whole lot better.
I, like you, cannot imagine end shake that egregiously large.
Perhaps if "speed loading", if the rear of the cylinder is not counterbored (right?) then the "relief" would allow a slightly protruding rim to breech and the cylinder to be closed in a lurch. Perhaps it's for the actual speed loader's mechanism to clear with the magnum length cylinder thus making this flat unique to the magnum K frames and not the .38 Specials.
My N frame, 25-5 , has no such "flat/relief". It must be something unique to the K frame due to its smaller cylinder/recoil shield diameter. Dunno...