Looks like you already found out it was an SR MTS-100 aka "slingshot" style stem.

I had one that came on a Univega Alpina Pro, looks like it may have been the same color (purple/dark burgundy) so probably the same year. Mine was from 1984, so I don't think that one is from the early 90s.

Edit: also, forgot to mention, moreso than bars being designed for a single clamp, this one has a different clamp diameter. It was made for bmx size bars (22.2), not standard mtb clamp diameter (25.4). I was able to put a 25.4 wald north road bar in there (8095 I think) with the bulged area centered in the stem and it worked alright, probably not ideal though. I think wald also offers a cruiser bar with 22.2 clamp diameter.