These fit inside a case and are run to the end of the cut for the case mouth/length?
We used to make these out of an aluminum rod turned down to the OD of the case at the head and to fit snug inside the case. For some reason, I never really used this info much other than to let case length in a .22-.250 grow a bit beyond max because I knew I could. The popular thinking had to do something with getting lighter bullets out farther or some such silliness at the time.
Eventually, I parted w/ that rifle and all the brass and bullets went to a friend. No good deed goes unpunished of course. Later, when he was using the cases, he came by my office and asked why I didn’t trim case length and didn’t I know that was dangerous? I explained the thinking and gave him the tool, but he was still shaking his head as he walked off. To this day, he tells people I don’t pay attention to trim! Oh well.
Such was life before you could buy all this stuff.