1943 D/S? Weak 4 Steel Penny

I think it's a reprocessed/re-plated coin with an S over S repunched mintmark.

Well it is a reprocessed coin (which ruins collectable value), but the mintmark is not a RPM. Why not? Because again direction Note that the mintmark wear is flowing towards the rim direction. Why does this happen? That is the direction that die wear shows as the dies wear. This year the Steel planchets and the steel die caused the dies to wear out a lot faster. It was not only the mintmarks that were affected, but the date also.
Looking at die DDO-001 you can see the die wear pattern start showing while even die scratches were still present:

Here are several images put together to show how the die wear affected this area:

Well I'll add to the information. Die wear on coins follows a certain pattern. The devices extend themselves in the direction of the closest rim. The means all devices. They start with the devices that are closest to the rim. Here is another example.

On the MDS (top image) the die is changing as it ages. So far the outer edge of the devices are starting to change from a squared device (on the side facing the rim) to be rounded over. On the LDS die state the round over is not removing part of the normal design. On the VLDS example that edge is loosing it shape because of die wear. Keep in mind the metal is being pushed from the center of the planchet into the voids on the die and stretching the planchet material to the rim. Thus the die wear starts first on the sides closest to the rims. On your coin the 'S' mintmark is the closest device to the rim and the shape of the mintmark is being distorted. As I mentioned these Steel planchets were more harsh than the soft copper metals used in the past. Thus what you are seeing is die wear on the outside edge of the mintmark. Not a different mintmark punch, just an altered 'S' mintmark. Hope this helps.