I believe it's a "metal piercing" round that was produced by Winchester beginning in the 60's. I believe it was used by some Highway Patrol in their model 27 and 28's.. I don't think it's been manufactured sincethe 80's.
Do markings look like this?
5cwRS.jpg
Edit: I believe they were tungsten core and Western also made the round.
Google- Super X .357 Magnum "metal piercing"
you will find some discussions.