Here's what I could find:

Cyrus Whittington Campbell (1887 - 1948?). Son of Cyrus W. Campbell and Effie Campbell (not the same Effie in PC, his mother passed 1888). His father had a long career as an electrician according to the 1893 & 1920 Kansas City Directory. Not much else on his family, my suspicion is the correspondence is with his sister.

Cyrus Campbell II married Blanche Colvin June 4, 1913 in Kansas City. According to the 1913 Kansas City Directory he was a salesman. In his World War I draft registration, signed June 5, 1917; he has one child, and is the "manager of the Douglas Shoe Company". Under previous military experience he indicates he attended 2 years at Wentworth Military Academy where he was a Private . The description on his physical is : medium height, stout build, with dark brown hair, and blue eyes. A Cyrus W. Campbell passed away in Kansas City November 1948, an article in the Leavenworth Times covers that but I can't find it to confirm it's him.

Just to make things confusing Cyrus & Blanche had a boy named Cyrus W. Campbell III. He was a student in Kansas City according to the 1933 Directory.

Good luck, and great pose!!!