I've never been very convinced by the explanation put forward by Barkman for the '13' mark...
With zero range the aim-point for both Spr and PzGr must be almost the same, with increasing difference with range. A constant and initial 5 degree aim off is just wrong.
Comparing the data given for the later sights shown below, Spr seems to share similar data as the MG34 (right side of the TZF12 diagrams), so using the MG scale and 'tick' on the left of the 9b seems logical and simpler.
Given the original reference to calibration, I prefer the assumption that the '13' tick mark is used in adjusting the mechanical linkages to conform the sight movement to the correct amount (given the Tigers operated in Russia where temperatures range from +40 in the south in the summer, and -30 in the worst of the northern winter, it would make sense to have some method of correcting for gross temperature changes).
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