Thor,
I think this is a scratch too. Since the mast could use some cosmetic touchup, why don't you sand the area and see how deep it is? Agreed on the post above - use some die penetrant, then look at it with a black light. Assuming it's a scratch, mask the compartment spray paint it to make it nice and shiny again.
I had the spreader fix done 2 years ago and had some real stress cracks spidering out from the rivet holes behind the spreader mounting bracket. The cracks were "opened" by grinding, then heliarc weld filled. I had Hall Spars do the work professionally, and had the "bar through the mast" fix. The spreader fix is in a much greater stress area than between the mast step and partners, so I don't think you have much to worry about. If you haven't had the spreader area inspected, I highly recommend you do that. We dodged a big one without knowing it!
Bill - Rhapsody J/30 #348