As a lefty, I have no use for a single trigger gun with the barrel selector as a button in the trigger. They are always set up for the open barrel with the button on the right and it is awkward to change from the left side. The only M21 that I own is a straight grip trap model with single trigger and extractors that I got cheap.
As left-handed shooter your ideal gun can not be large factory or mass-produced. What you need is shotgun with appropriate cast, trigger arrangement and lever that operates right to left. That requires custom ordered gun or second-hand one made that way for somebody else. I'm sorry to tell you this but Winchester 21 isn't that shotgun.
