I'm categorizing and boxing my Universalist ephemera -- remember the Hollinger boxes? -- and found a cache of the Sunday School Helper "devoted to the interests of Universalist Sunday School Teaching."
Some time in 1877 or 1878, the Universalist Publishing House moved in Boston from 37 Cornhill (a district with a long history of Universalist publishing) to 16 Bromfield Street. I've probably walked past this place because it isn't too far from King's Chapel.
The place is now an Irish pub -- the Dubliner -- which seems to be a good reason to visit.
16 Bromfield Street (Google Maps)