I checked other examples on PCGS and they have similar full and partial missing initials that was graded, wasn't sure if this would be an example
I read somewhere that this happened when they polished the original die to make proofs. Might be a proof!
The initials on the Walking half are incuse and in the field. The field of the die is normally the highest part of the die, but to make incuse initials the initials have to be raised making them the highest point, so what happens if you have a die clash and have to polish them out? It tends to polish out the initials. Frankly I am very surprised there aren't a lot more walkers with missing initials.