You had a little extra time to come up with an answer to the most recent Philatelic Q & A, but unfortunately it appears that the answer wasn't on the tip of your tongs.
The important modern cancel that emanated from a post office over 100 years old; was the first USPS pictorial cancel ever issued on Sept. 27, 1971. It was applied to mail items in the Headsville West Virginia post office to note the establishment of the old post office in its new location. The building seen in the cachet and cancel on the cover above is the very post office where the cancel was applied. The post office had been packed up and moved to the Smithsonian Institution for display and use for many years at the august site before the peripatetic PO made another move to the American Philatelic Society headquarters in Pennsylvania. If it keeps moving I think it may have to be designated a mobile post office someday.
Smithsonian cover from collection of the author; Stamp and cancel images © USPS