I hit the motherlode today: I found both a 1910 stencil catalog by Sherwin Williams and a 1913 stencilling guide by Frank Gibson. (I actually found so much more, but this is all I feel motivated to post right away.) Anyway, I'm going to share my favorite stencil designs here for those who are interested.
This is the set I like from the Sherwin Williams catalog:
The Sherwin-Williams stencil colors.
The Gibson stencil colors.
I also ran across an Alabastine Paint stencil booklet a few weeks ago. I may as well share some of them here as well.
And for completeness, here's a few more from a Devoe stencil and paint supply order form.
Update on Sunday: Just found a few more from a Wards Paint catalog from 1915.
And here are a few more from a 1912 Sears catalog.