Thank you Bernhard. I found a few references to Breitkopf edition no. 3765 on the Web, but little information is given. For example, from the Sheet Music Plus website: "By Johann Sebastian Bach. For SATB. Published by Breitkopf & Hartel. (912002090)"
and from Breitkopf's site, all it says is this -- "389 Chorales with instrument obbligato - mix ch,pno"
Does this mean that the piano scores are an exact reduction of the accompanying SATB scores?