I found Frank listed in the Twelfth Annual Report of the Board of City Service Commissioners of the City of Milwaukee on Google Books. I bet it’s an exciting read, but I was more interested in seeing exactly where he worked.
He is listed on page 53 as being part of the “Appointments in the Labor Service During the Year 1906” in the “Parks Department.” It says he worked at Lake Park, which you probably guessed right, is next to Lake Michigan. Frank didn’t live too far from the lake, so this makes sense.
The book contains mostly statistics and rules, but I was lucky to find Frank’s name. It pinpoints him in the year 1906.