I know they were enhancing the neckline to strengthen the design. But don't know if this was a hub issue or a die issue. If a die issue, then there would several others from that die that would have this. If there were an enhancement on the hub then there would be several dies that would show this. I've not seen that enhancement before. I've seen other examples:
It does look like engraving. So we will have to keep an eye out for more of these examples. In 1968 the design on this master hub had swell outwards so much that that the outer devices were touching the rim. So they created a new master hub in 1969:
Note the devices have moved away from the rim in 1969. But also note the bust sizes are different. The older master die had swollen that much over the years. New master hub, normal devices are back again. I would save this for a reference coin to see if more of these show up. (why would that be helpful?) Because the mintmarks are in different locations, if we find another one, with a different mintmark location, then we would know it is an alteration on the master hub. But if we find only other examples like your coin, they may have alter just one die. So save this and see what turns up in the future.