Hi Marcus,
The distance between the touchole and the rear is practicaly the same as that from the bore bottom to such rear (9 cms.); only a couple milimeters difference.
Further down from the 'conical' entrance to the touchole and its 'eliptical' worn vent, there is a very regular 4,5 m/m orifice.
Always a problem to take pictures of a barrel bore ... at least for me
.