Thanks for the reply. You could perhaps say that, because the blob is raised above the rest of the coin, the pattern has worn off it. However, because it is raised, you'd think that it would have prevented the rest of the obverse—certainly the part immediately around the blob—from being struck and there's no evidence of that, so it is most likely something that's happened post mint.
If you attached (How? Welding??) something to the coin in order to make something, why would you then hammer it hard enough for the thing you attached to show through the coin? And if you were making, as you suggest, a button, wouldn't you want the reverse, rather than the obverse, to be the visible side?
I'm still flummoxed.
Thanks again.