Check out the recently new post in the FAQ's.
http://parkerguns.org/pages/faq/PatternTest.html
I'm guessing but Parker may have changed over to the 30" at 40 yards around the late 1890's or early 1900's. Maybe with the introduction of the hammerless guns. I'll see if I can narrow it down more. The problem is that most of our copies of the stock book are cut off right at the middle of the patterning info. I need to page through to find some that can be read.