Hello and thank you for your examples and hint. Indeed I had also my doubts about the color, more as about the type. But somehow the color is consistent all over the stamp. You're right, there are darker spots, but those and the less dark areas still have the same color. Even with microscope I don't find any area in the stamp where I could find a more red oder lighter color shade. I made here another picture with camera and adapted the color in Photoshop so that it fits the real stamp quite well, on a calibrated monitor with color profile embedded in the jpg, and with activated color management.
But I never had these difficulties and different color shades in scans and photos like with this stamp. At least it shows that this stamp is not an easy one. As said, it's not sure that it's a 319a, but possible and more likely as with all the certified ones.