Looking for any information on this cover that I purchased on ebay, postmarked April 1, 1939, at Opportunity, Nebr. (Seller’s image.)
The cover does include the words “Cachet by Williams” but that didn’t give me much to go on, and I haven’t figured out who Williams might be.
Likewise, research on the addressee, Joseph Piltch of Brooklyn, turns up little more than the dates of birth and death (assuming I’ve got the right Joseph Piltch).
My best guess at this point is that it was created for April Fool’s or for the town’s name or both. I don’t think it’s a FDOI because the stamp is US Scott 798, which was issued September 17, 1937.
I don’t think it’s a first or last day of operation, either, because the post office was established well before that and was not discontinued until late 1942/early 1943. The Postmaster at the time was Roy Alder, and I haven’t turned up much on him, either.
I was able to find some information on the town of Opportunity, although there are some discrepancies:
Opportunity was established in 1911 when a man named Cox founded a cheese factory there. The town reportedly got its name because Mr. Cox said "the manufacture of cheese affords the community farmers an opportunity to get rich off their milk cows" (Yost p. 370). Unfortunately, the factory closed sometime near the end of World War I. The town survived until the depression years of the 1930s, then ceased to exist.
from Nebraska Historic Buildings Survey, Reconnaissance Survey Final Report, Holt County, Nebraska, prepared for Nebraska State Historical Society State Historic Preservation Office, June 1, 1988
…settlers felt that the area near the Redbird Creek, on the star mail route from O'Neill to Dorsey, would be a good opportunity for successful businesses and a post office. Hence, the office was named Opportunity in 1910. The settlers' premonitions proved partially correct when a cheese factory flourished in the community for a few years, and the post office served the area for thirty-three years, thirteen more than the county's average post-office life span of twenty years.
from Buller, Rebecca A., "Life and Landscapes In The Post-Office Communities of Holt County, Nebraska" (2008). Great Plains Quarterly. Paper 1345