My guess would be that, fifteen month old's actions being universal, he had
to bear down quite a bit to get the marks to take and you are looking at
depressions with the ink down in it rather then sitting on the normally
smooth top surface.
Try a soft tooth brush and some paste wax and see what happens. If that
works all you are left with is the dug in marks from the pen. Lucky you.
Click to see the full signature.