I found this piece interesting and thought I'd share it. A postmark from a place called Harding, Massachusetts, that is no longer carried on any map, nor is it recognized by the US Postal Service as a legal city/state address, yet according to Jim Forte's Postal History Website the post office was only closed in 1972 (which is relatively recent when researching postal history) and Harding, Massachusetts even had their own ZIP Code (02042) which is also discontinued:

Come to find out the area is now incorporated as part of Medfield, Massachusetts. Apparently Harding, Massachusetts dates back to when the now-closed Medfield State Hospital was in operation.
A local newspaper did an interesting historical account of what was formerly Harding, Massachusetts at this link. I thought I'd share it for anyone who may be interested as I find it rather curious that there are very few other references that can be found to that now discontinued post office location:
http://medfield.patch.com/articles/...f-medfield#c