Because the spot is "right on the (faint) mold seam," I believe your guess is correct, it is a patch-job for a significant casting flaw.
For anybody who doesn't already know:
The mold's filler-hole is always located on the mold's seam. On occasion, contraction of the cooling metal -- or an air bubble trapped in the molten metal during the mold-filling process -- causes a sunken-in spot on the projectile at the mold's filler-hole. If the casting-flaw is of a size large (or deep) enough to possibly result in fracturing when exposed to the explosive blast of the propellant-charge during firing, the shell-manufacturer would "patch" the flaw by filling it with an easily-malleable metal, such as lead ...or in this case, copper. Because a malleable metal is used for the "patch," it could simply be hammered into the flaw, instead of having to use a more complicated repair process ...or having to scrap the whole newly-cast projectile.
Regards,
Pete