The H-5000 was the model name for the Husqvarna 1600 (HVA) series rifles imported by Tradewinds in SeaTac, Washington, in the 1960s. The HVA action is a small ring commercial 98-style action that Husqvarna built their CF rifles on during the 1950s and 1960s.

The HVA/1600 series actions are probably the most common small ring commercial Mauser-style actions found in the U.S.

To make things confusing, after Tradewinds lost their import agreement with Husqvarna, it went to S&W, they imported Krico rifles from West Germany and called them H-5000s also.

"The large plastic lower unit" that you ID'd sounds more like a Krico than a Husqvarna to me. The Kricos have a detachable magazine, while the HVAs have a metal, either alloy or steel, hinged floorplate assembly.