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fkwvertical strips of 10 I believe. I have 2 10 card vertical W573 strips and I believe they were issued the same way.
I also have a theory that they (along with W501s) were also torn from some type of holder sort of like scotch tape. Many of the cards from these sets (especially W575-1B) have rough rips (some angled) on top, bottom, or both.
Rhetts page is the best place to view some (W575-1B), and I believe he also knows the most about these related sets.
http://www.starsofthediamond.com/e121.html