material placed in a new shoe to help keep its form and display it expanded as if being worn
A New York store introduced a plan of placing fillers in each shoe as it was put in stock, these being sold to the customer at a cheap rate to use as shoe trees.
…
Filler are generally used in shoes carried by traveling shoe salesmen to keep their samples in shape while being handled.
— The Shoe and Leather Lexicon, Fourth Edition, 1918