There was an article that appeared the Watch Tower of June 1885, entitle ""To the Elect of God (In a Time of Trouble)", written by Robert Roberts. I have not been able to ascertain if this is the same Robert Roberts that wrote "Christendom Astray", but it appears to be so.
I have a copy of a 95 page A5 booklet titled "Letters to the Elect of God in a Time of Trouble" by Robert Roberts and it contains four such letters and other material, some added by the editor of this edition. I could not match the Letter that you referenced with any of these four, but Robert Roberts was the editor of The Christadelphian Magazine and the article quoted may have been an earlier Letter in that magazine. Some of Russell's assessments of the Christadelphian beliefs are not accurate.