Let's start at the very beginning...

1879 - "Rockland" Residence of Henry R. Towne

E. G. W. Dietrich's watercolor of the
 H. R. Towne residence, held by the CMOA.  

The earliest known architectural drawing by E. G. W. Dietrich is this beautiful watercolor of "Rockland" the Henry R. Towne residence in Stamford, CT.  It is signed simply E D. 1879 in the lower right corner.  Mr. Towne's house was designed by New York architect H. Hudson Holly (1832-1892).  The house was published in the American Architect and Building News on November 8, 1879.  Likely made as part of Dietrich's architectural training, his watercolor exactly matches the viewpoint of Holly's published drawing. 

H. Hudson Holly's design for the house of
 H. R. Towne published in 1879 

c. 1892 photograph of "Rockland"
the H. R. Towne residence from Picturesque Stamford