A D/S over mintmark (or OMM), would likely show on the outside of the D, not just the center. Just because you see some diagonal stray lines, doesn't mean it's an S. You need to determine if the size and shape is an exact match to the S mm used on 1970 nickels. (pulling out my inner @Insider using colored text ) It's more likely just a stray line caused by die wear, die polish of minor die damage. those are much more common occurrences than an OMM
That's how I learned, Sal, from mistakes. I figured it's better to listen and learn than to continue to make them, so I listened and learned, what the hell?
I'm far from being an expert in anything, but in my humble opinion, it appears to me that the post on the die that forms the center of the D mint mark, is in the first stages of deterioration and has caused the anomalies you are seeing. Just my opinion. Would love to hear @Fred Weinberg or other experts opinions.