Purely speculation, but based on the idea of 'calibration' I'd hazard the mark at 1300m is used thus:
The tank is levelled, and the gun aimed at a panel or another Tiger 'through the bore'. The sight is adjusted so that the aim point remains on the target and the range reads zero. When the range is increased to 1300m, the sight aimpoints should move down the panel/Tiger by 5 degrees, which could be a second mark or e.g the ground at the track at a known (measured) range. The linkages are adjusted until this is true, and then firmly locked, giving correct adjustment for 0 and 1300m, and thus for most combat ranges.
It is possible that a zeroing round would be fired to confirm the boresight, but this is not always required, especially if the crew is trained to fire using Burst On Target or similar techniques.
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