Terry, at this stage I have to say I do not know if that SN is factory or not.

I started by comparing it with my 1957–59 Sportster examples but that isn’t many. The lack of a serif on the 7 is apparently okay although it seems H-D may also have used a seriffed version for some Sportys around that time because I see it in the sequence portion of some of my examples for 1958–59, and for some other years. Although the back of the 7 doesn’t appear exactly the same as my examples below, it may still be alright. As I would expect, the letters are both sans serif. The strokes of the X cross each other in the centre, more or less, and the horizontal stroke of the L looks about the usual length. The 1s appear to be okay.

The 5 and 3s: Even after expanding the search to include my 1960–69 Sportster photos I could not find a match for the 5 as far as its lower half is concerned. The 3s are sans serif and that is apparently normal. But notice how far the bottom of the 3s extend to the left when compared to the top. And that extension, at least in that type of 3, is something I cannot find a match for even after searching my 60–69 photos. And part of the reason I say that type of 3 is because the end of that extension, or arc, is very low when compared to the 3 in my example below.

I can’t see any real sign that the boss has been altered but of course better photos would tell us more, including how thick the boss is.

Did you check for belly numbers? The code number should be 7 so an example would be 757-1234.
Eric