Imperial was absorbed into Schrade, but I don't recall when.  The company was know as Imperial Schrade for a lot of years until the name was shortened to Schrade.  I do know Imperial existed for quite a while before the merger.  I've an old faux boy scout knife that dates back to when I was a kid in the early fifties marked Imperial.  kA-BAR also used that blade design.  I've an old Ka-bar skinner dating also to the fifties with the exact same blade, and that model wan't new when my dad bought if for me.