The thread of a Mauser is actually 12 TPI with a 55 Degree thread angle which is Whitworth, a metric thread was never used.

The thread in the picture is not a square thread but a normal V-thread with the crest of the thread polished which contrasts nicely against the rest of the blued thread.

On take-down rifles using modern actions the thread used is often an 29 degree ACME (if inch) or 30 degree Trapezoidal (if metric)especially if the rifle has a segmented thread for quick detachment.

A thread with a V form is used as with a precision cut thread the radial run-out of the barrel can be removed as the threads self center by the mating tapers of the threads.

Matt.