Before someone knocks this as off-topic, I'll put up a couple items. I collect old sports letters, so that's all I have to show. These letters are probably better classed as semi-pro rather than professional.
1933 Denver Piggly Wiggly - a dominant AAU team that eventually evolved into the Denver Nuggets
denver.JPG
circa 1920 Belcur basketball team - I don't know anything about this team. The letterhead is blank.
belcur.JPG
1901/02 St. Clair Athletic Club - According to web reports, this team was fantastic. The player with the mustache is John Titus, who played baseball with the Phillies.
st_clair.JPG