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Also, it may take a week of daily shampooing to get the doggy smell out of some brushes.

I also recommend the inexpensive "Omega synthetic brush." under $15
~Richard

Just thought of somethings else...
Old style scuttles were in use for a long time and the lip of the reservoir was fairly small. so a small diameter brush would fit them.
278105
and...
Most...

Yes,brushes were often smaller whatever material they were made of.
Usually boar and horsehair, which mostly went away due to Anthrax scares after WWI, became a bit larger than badger in the 40's....

This is just my take!
Diameter is for coverage and lather capacity.
Loft/brush height seems to be related to stiffness.
Old brushes were usually 20mm-7/8ths diameter. New seem to be whatever brag...