They were used for reloading Remington SP (steel plastic) hulls up into the early 1970s.
They are smaller than the 209 size primers. Most people just bought 209 primers and forced them into the Remington cases the first time.
After that 209s could be used. I bought them for a while. Then I used up the last I had and continued using 209s until the Rem SP cases went into the trash.