Normed histograms for the coordinate uncertainties of stars, including only the uncertainties related to epoch transformation (Gaia) or source extraction (CduC). The histograms are indistinguishable both along RA and Dec. The red line is for the coordinate uncertainties in the single-exposure plates, as given by SExtractor. The green line is for the coordinate uncertainties in the triple-exposure plates, as given by our fitting procedure. The blue and orange lines are for the coordinate uncertainties of the Gaia DR2 and EDR3 stars at the epoch of the plates, respectively. The vertical black line gives one-tenth of the pixel size of our digital images.