The explanation is that it is a mistake made by the transcriber. If you look at the directory itself it shows that William F. Boswell works for U S A. It is then followed by his home address. The odd transcription shows for both 1918 and 1919.
Their *transcribers don't have the concept or a perception of where a person's name stops, and an employment place begins. I have even seen clk (meaning clerk) inserted into someone's name & in the directory itself, the person's job is working as a clk at particular company.
*or the directories were done for ancestry by use of (OCR) optical character recognition.
I hope this helps!
(USA may be initials for the company he worked for or it was the USA - he worked for the federal government).