Can anyone confirm that the Church of the Assumption at W 49 St and Ninth Ave was considered a German parish? I am interested in the period of the 1870-1900.

I would suspect that it was, based on the following excerpt from a long article in the NY Times of December 25, 1881:

The German Catholic churches have all been decorated with Christmas greens . . . .

. . . . Great preparations have been made to celebrate Christmas as far as possible in the fatherland fashion in the Church of the Assumption, in Forty-ninth street, between Ninth and Tenth avenues.
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Another positive indication:

NY Times
Nov. 9, 1922

The Reverend James Veit, 60 years old, pastor of the German Roman Catholic Church of the Assumption, 427 West Forty-ninth Street, died in Roosevelt Hospital last night . . .

Thank you very much.