Square primer pocket on .38 Special blank

I recently obtained what appears to be a .38 Special blank cartridge in all ways except that it has a square primer pocket with a large round primer in it. What is the purpose of this cartridge and what is the function of the square primer pocket? There is no headstamp.

Hi, Probably a Western Cartridge Co. 38 Special blank, for a training mine designated 3F15.
(per Fede 9-18-19?) Here is a sectioned pic from Pete.

Dan, I will have to express my ignorance of such things. What is a “training mine 3F15?” And what relevance is the square primer pocket? Any education on this topic would be appreciated.

Good questions, but I don’t know the answers! see this post: