There are lots of English guns that will fall in that weight range esp. boxlock non-ejectors, which tend to be lighter than sidelocks (and cheaper...).
For $1500 you should get a clean plain English gun. If you broaden your search to include German, you will get a clean fancy gun. Belgian, you will get ditto for $1000; and Spanish, for $750.
Of the American guns, you may find a Fox or an Ithaca that light but most vintage American 12 ga guns are heavier. Would pretty much have to be a lower, the way things are going. You might swing an A grade Fox for that money though.
Before I went for a 12, I would think about getting a 16 ga in a lightweight though...it'll be a stronger gun and it won't kick as hard. Important for fast second shot on grouse.
Surprised you find a 20 not enough gun. For close grouse work, 20 should be plenty. Maybe it's a function of fit?
