This brass cased Spanish 7.92x57 Round is headstamped “FNP 7.92-955”. It looks to have never been fired, and the primer is unstruck. It shows aluminium deposits from a puller collet in the extractor groove, and a black plastic tubr inside the neck, which I presume is where someone tried to pull a bulleted blank or short-range ball and the bullet broke off. Is this round a bulleted blank or short-range ball? Also, is “955” the date (1955) in Italian style?
Here is a picture of a Spanish M1957 Blank which has a black platic bullet. If it were broken off at the mouth of the case it would leave a tube as you described. My records indicate that the M1957 can be found with 955, 956, 957 dates on FNP cases (also with P 58 7,9 headstamp). Maybe this is what yours is.
That’s the one. Now just to find a complete bullet for it to make a proper round. The primer on mine is crimped in with 3 stab crimps.