Henry F. Hammer
1885

Updated by William M. Worden, who gave this as the source of the information: General knowledge of Detroit architects..
Peter Dederichs was the architect of the present St. Mary's Church and had a long career as an architect beginning about 1873. As far as I know, he had nothing to do with organbuilding. In terms on bona fides, I was the City of Detroit's Director of Historic Designation from 1977 to 2006. A notice from the Free Press identified the builder of the first organ in this church--I think it was Hammer. There was no indication in the article that the organ was moved, but that could have been the case.

Updated by William M. Worden, naming this as the source of information: Detroit Free Press as noted below.
Detroit Free Press for April 13, 1884, confirms that the organ was moved out of the old church "and will be rebuilt for use in the new one." The Free Press Aug. 17, 1885, does name Henry Hammer as the builder.

Updated through information received via e-mail from Dianne Little, archivist of Old St. Mary's Detroit:

I know that the original organ was used in the first church built in 1841-42 and the organ was placed there. When the old church was torn down to make way for the Peter designed church the organ was moved to the school and services were held there until the new church could be built. The organ was then moved back into the new church in1885. The church has a wonderful archives that date to that time period but no where have I been able to find info on the renovation of the old organ let alone the addition of any stops etc. There has been no info on Peter, the architect, doing any work on any instrument or organ in any other church and he was known as a church architect and having designed over 45 churches. Peter's father, Peter Sr. was listed as a cabinet maker in the 1850 census but in later years in the 1860, 1870 and 1880 Sr. was listed as a carpenter.

Identified through online information from James R. Stettner. -- Peter Dederichs was a parishioner at St. Mary's. When the new edifice was built in 1885, the existing organ was relocated with 3 new stops and "some mechanical and visual enhancements." It is listed as being 25 stops. It was replaced in 1928 by a new 4-man. Geo. Kilgen & Sons instrument. Disposition of the Dederichs instrument in unknown. Source: church website.