World Inequality Database: Top 10% Income Share U.S. vs. Top 10% Average Income
I have actually found an answer to my own question. In case anyone's interested, I found the data on the top 10%'s income share going back to 1913 on dr. Mark Frank's website - he collaborated on the construction WID. I suppose the series have gone missing / were excluded for some reason on the WID's platform? 

https://www.shsu.edu/~eco_mwf/inequality.html