Interesting. My GOEX plastic cans have black plastic caps, just as the Swiss can had. I drilled an hole in one and used CA glue to glue in a .308 (or other??) empty
case with the base cut off. This is for filling horns. As I has some small horns with small calibre pouring holes, I did the same with another using a cut-off .223 case.
Works fine with both GOEX, Old Enysfor and Swiss cans.
The older small metal top for metal cans also have the same scenario, but with the brass cases soldered on.