I did a little research however will certainly need to send way to NPRC to confirm.
There were a couple soldiers bearing the same name in BIRLS and Draft Registration files on Ancestry. I was able to find 3 newspaper articles from July 2nd, 1918 with casualty lists identifying a Joseph Schlachter from Rock Springs, Wyoming as being severely wounded. I'm fairly certain this is the same soldier this Heart belongs to.
Below is one of the articles (all three have the same details which is a press release from the military), this one being from the Kingston, NY Daily Freedom:
Only other record related to Joseph's military service is his WWI Draft Registration Card:
The remaining info I could find was Joseph and his family in the 1930 and 1940 Census. In the 1930 Census, he is identified as a veteran of the World War:
Lastly, here is his SSDI file: