More of an aging die cap that is deteriorating, thus filling/preventing the devices on the die, so that nothing can rise on the struck coin. Note how this happens on the extra images I just posted. As mentioned earlier, there would be a rim on a
Struck Through Grease, The cap prevents the rim to form, because it is covering the gutter on the die. The more the die cap deteriorates, the more of the rim/devices/design will eventually be seen again on the coins struck with that covered die.