This 9mm round by Karlsborg Munitions Factory has a red bullet that I’m sure isn’t metal. Headstamp " 5 " 4 " K " . Is it a dummy or blank?
What you have is a plastic bullet blank. It was made for the Swedish military using reloaded/reject cases and was in use for many years. You can see a cutaway of the bullet at gigconceptsinc.com/Misc-9mmP-Data.html, scroll down to the second entry.
The Danish Army used it also after 1945. Partially on Swedish cases (or imported cartridges) and also on reloaded ones like the “pjj” and British WWII as well as Danish cases from 1942 and later.