I had never heard of Die Sinker's Riffles until this weekend, and then, when looking in my garage for a caliper and micrometer to bring to work, discovered that I have a set!

the origin of the term, is of course German, and the translation is basically: files for making die molds more perfectly shaped or formed to stamp out metal parts 

So, die (part making metal mass that is pressed over a sheet of metal) sinker (down) riffles (files)