That is a Magnum LST.
Here is my Magnum Jr. Really solid, and fits my medium-size hands well. When I inherited it, it really got me into lightweight lockbacks. For the money, these are great.
While most were black, the Buckmasters specials were dark green.
They do take and hold a great edge. Steel can't be 420HC as it holds a great edge. It sharpens and works more like 440C.
I think when the MP line came out, the numbers changed, and I believe they were retired finally in 2010 or so. The box that came with mine lists it as a 400, but it's stamped 500.