That's what it looks like to me a copper wire soldered through the date. Now the real question what was Einstein trying to achieve ?
I did look at it. Weird. But I think it's just some strange soldering experiment. Struck through would be mainly into and below the surface. I disagree on the Die Crack.. They aren't.
It does look like a bad attempt at making a fisher. Back in the day, before all the modern do-dads in the coin op mechs. when working in an arcade most would drill a hole in the edge of the coin, and solder a spring wire commonly called piano wire at the hobby stores. As it wouldn't break as often as a copper wire of that small of dia would. When it was used for a free play, or the machine didn't give the patron his moneys worth of play. The idea I was told kept losses down. As you would hand off the drops to the next floor person, and not forget to give back the pocket of change back to the cage/change teller.I think I still have some around here from the good ole days will have to hunt them up and photo them.
It could be a strike through... wire brush piece fell on the planchet and was embedded in the striking process....
I have no idea whatsoever could have happened, but I would be very interested in what @Fred Weinberg or @physics-fan3.14 thinks.
I will say, when I was a kid my parents bought a house. During the inspection we discovered all the fuses had a penny with a tiny amount of solder on behind each fuse. Just my guess. not an expert.
The wire though the date looks correct but why someone would do that is beyond me. I once found a mercury dime cut in half sideways and cemented to a Lincoln cent. I took the two apart an had 11 cents. Yes....and I did spend them! The dime was not silver but Aluminum. Why people would go to such trouble to do this? I don't know but the fun is in finding stuff like this!
My question is, is there actually a raised wire? the photo to me looks like a deep cut that pushed the metal up towards the top with more being pushed up at the raised numbers. a response of raised or incuse from the OP will solve it for me.
I'm also having trouble seeing a wire and was thinking a deep cut with displaced metal as well. Over time and circulation, the displaced metal was worn down, leaving some flattened areas. The numerals in the date are going to take most of the wear, so that's where the metal is flattened the most. The bright areas in the pic could be a reflection caused by the photo. In addition, I don't see any solder remnants or heat staining. If the flattened areas on top of the date is left over solder, then the person must have been a talented craftsman to use such a miniscule amount. And if there is a wire on the coin, forget I posted this