According to this list, it's a 1931. There is tons of info in the Wisconsin Historical Society archives, but some of it takes some digging to find.
http://content.wisconsinhistory.org/cdm ... 511/rec/16Click on 1928-1934 in the right hand column.
The numbers in the chart are the beginning serial numbers for that year.
Here is some other information that might interest you, also-
http://content.wisconsinhistory.org/cdm ... 220/rec/65Click on A2 in the right hand column ( 2 pages).
Hope this helps-
Dean