The 'Y' looks like a a struck through debris onto that devices. Coin wrapper damage on the head. Not listings of a DDO on 1988-D on Variety Vista:
http://www.varietyvista.com/09b%20W...Listings.htm
Nothing on Wexler's site for that year either.
On the motto, you can see how die wear increased the size of the outer devices. Note the inside ones, then the outside. So just die wear is a factor on how the motto was altered with die aging. It is were hub doubling, the devices would start out being the same size, but when the die reached the same age as this, the doubled die devices would still be altered like this coin, but would have the doubling, but also be enlarged as well. Just what dies do when they age.

http://www.varietyvista.com/09b%20W...Listings.htm
Nothing on Wexler's site for that year either.
On the motto, you can see how die wear increased the size of the outer devices. Note the inside ones, then the outside. So just die wear is a factor on how the motto was altered with die aging. It is were hub doubling, the devices would start out being the same size, but when the die reached the same age as this, the doubled die devices would still be altered like this coin, but would have the doubling, but also be enlarged as well. Just what dies do when they age.



























