From my postal history collection, I will share with the club a registered cover mailed from Firenze, Italy on 28 January 1911 to Des Moines, Iowa, USA passing through Chicago, Illinois on 9 February 1911 with United States Post Office Label - Form "B" attached to the front, titled "SUPPOSED LIABLE TO CUSTOMS DUTY." The cover arrived in Des Moines 10 February 1911
I searched the Stampsmarter "United States Post Office Forms Database" for Form B and found it listed, but without image. Don D, you are free to use my images below for Stampsmarter if you like, as well as anyone else who can use them. Stampsmarter listed the EKU (earliest known use) as "1912 DLS." I believe this label would have been applied in Chicago on 9 February 1911 on my cover.
The label measures 39mm in width at the top, and is 71 mm tall.
Using my Linn's Multi- Gauge, the label is Perf 12 on the left edge.
The letter "L" is omitted in the word "located" so it reads "ocated." The letter was not scuffed off, it was simply not printed at all.