This at first seemed to be a one-of-a-kind cornet with an unusual way to plumb an Eb. The wide flanges on all the braces is distinctive and the thin finger buttons suggests a date of 1880 or earlier.

The bell diameter is 4-3/4" and the length is 10-1/2"

On doing some research I was able to find out a little about this maker.
1865: Whitmore, Thompson & Co. Equescurriculum
1868: Whitmore & Co. Hippocornique
[Apparently he was involved in the circus.]
1872-3: O. A. Whitmore listed as musician in the Boston city directory at 103 Court St.
1872: Eben B. Mansfield is in the Boston directory as a banjo manufacturer located at 103 Court St., the same as Whitmore.
1875: Whitmore is in the Boston directory at 282 Washington Street.
1879: Whitmore moves to #178 Washington St. and runs an ad for "cornets of my own manufacture developed after two years of experiments after models of Besson and Courtois and made to special order for me in Paris."
1880: Whitmore & Boris, musical instrument dealers & makers
1880: Eben Mansfield and Whitmore share patent #229,622 for a music stand.
1881-82: Whitmore is a clarinet player for the BSO.
1882-84: Whitmore runs ads for a music shop in Boston.
1885: both Whitmore & Masfield are gone from the directory

My guess is that this cornet dates to the late 1870s when Mansfield was his partner and he was having these made in Paris.

This is as I bought it on ebay.

Here is another cornet I found on the Horn-U-Copia website that is marked with the Whitmore name. It does share some design characteristics, such as the receiver socket, lyre holder and wide flange braces, so was probably done by the same maker. Since this one only has the Whitmore name, it probably dates to the early 1880s when he had the store. The bell reads "Courtois Model Made Expressly For E. Louis Eager by O. A. Whitmore Boston".

Above is another cornet marked "Whitmore & Mansfield" that I saw on ebay in 2014. They wanted too much for this wreck so I didn't even try for it but this also has the same wide flanges and lead pipe socket.