Great that you posted those pictures - I don't nearly have the same selection and it illustrates what I'm questioning very well:
picture 1: small font, no US marks (I have a rifle like this one)
picture 2: small font sanded, large font number stamped later, US marks
picture 3: large font, US marks
Also, am I correct in my observations that the small font is usually 3 digits and large font - 4?
I was wondering about the small font numbers. How common are they? Do they consistently appear on rifles that have no US marks? Are there known Canadian rifles with no US service but with the small font number? Or other way around - are there known US service rifles with US marks but only the small font number?