You said that a similar 9x19mm case had been found. What is the headstamp?
I have a 9x19mm headstamped
Which is Polish military. I also have it in 94, 95, 98 and 99 dates. It also occurs without the “.” behind the caliber with 93 and 04 dates.
As far as I know this headstamp is used exclusively for Polish military cartridges in 9x19mm.
Your 5.56mm headstamp has two dots that are not on any of these headstamps so it could be something else. Were these two dots on the 9x19mm headstamp you referred to???