I'd really reccomend taking a look at it closer from a different angle. The flat-on microscope can skew your view, and I've seen something like this on pennies a lot. It's usually just the light bouncing off both the base and the peak of the mintmark...
I'm really probably wrong, you almost certainly know more than me about this!
Nice mercury, by the way. I have three very bad quality
Mercury dimes, I found them in a field about 3 years ago while digging up a garden