42d Art Forms. It’s a bit opaque. “Art for” is an archaic representation of the second person “thou art for”, I.e. you are in favour”, though I don’t like the you being omitted. MS is writing (manuscript) and music, sculpture etc are the definition
21a As Of. A sofa minus the indefinite article. It means from (a particular time)
It’s KNEE-SWELL. Some organ require the player to operate the bellows with their feet, so swell is controlled by a spring-loaded lever next the the players knee. Not easy to use!