"My assumption is that the mapping for the census was done well before the 1874 realignment and they were working from the 10 EDs then in existence."

Thanks very much for your research and informative post...and I believe your assumption is on sound footing.

For the record, this issue arose after reading what appears to be a correct but apparently unclarified remark referencing the 1875 Census at the "Brooklyn Genealogy" website (a terrific asset about which I'm sure you're familiar) in which it asserted that the "districts" used, specifically, were "election districts". While essentially true, their "note" probably should be clarified by the addition of the information you've unearthed...that the "election district" boundaries utilized for the census were not those in existence at the time the census was taken.

I've also posed this question at the FamilySearch forum ( https://www.familysearch.org/learn/forums/en/showthread.php?... ) and, with your kind permission, I'll append your information to that thread (with attribution of course) unless you'd prefer to do so yourself. This might also be worthwhile information to add to the 1875 NY State FamilySearch wiki ( https://www.familysearch.org/learn/wiki/en/New_York_1875_Sta... if you're so inclined).

For reference, here are the 10th ward boundaries as published just prior to the November 4, 1873, elections... http://eagle.brooklynpubliclibrary.org/Default/Scripting/Art...

Again, thanks very much.