I was looking at the 1879 map of Charlottenburg Twp. and it appears that for some reason there was a departure from the standard of naming lots with Arabic numerals in favour of a letter system in at least one isolated area. Perhaps some other member knows the reason for this.
The "D" doesn't seem to signify anything other than its normal place in the sequence of the alphabet.
I've attached an image of a section of the township where this alternative alphabetic lot naming system was used.