It is the Archive listing of the Smithsonian holdings!!!
I love that sucker!!
That is how I found the titles of some of the WU Books and Items in my collection!!! 
The WU Teleprinter Maintainers Book that I got you a copy of and My Postal 
Telegraph Company teleprinters Books were all found with the aid of that index!!
I have some other WU titles that are not in the book too, along with some Local WU stuff
from Downtown and the Train Station here.
A lot of the Smithsonian collection are from the former WU museum and from former
WU Employees. 
A very cool nugget!! 

JD