According to Mary Ellen Brown's book, the Part I edition was sent to subscribers by the middle of December 1882. She has nothing to say about the subscribers themselves (afaik), but her bibliography lists the Houghton library at Harvard as holding the Houghton Mifflin Company correspondent and records 1832-1944 and the Houghton Mifflin company records guide. It's possible the list of subscribers is amongst these.

Mick