Sam Denby from Half as Interesting, in his distinctively frantic manner, explained how and why the JW Wescott Mailboat on the Detroit River was given its very own zip code (48222) in 1948. Denby explains that this non-military floating post office in the United States addresses the high traffic of commerce that travels up and down the Detroit River every day.
The freighters on the Detroit River, however, couldn’t be assigned a ZIP code, since there was no post office floating in the river. So, to remedy this, the USPS granted the J.W. Westcott mailboat the rank of post office and gave it its very own ZIP code: 48222—essentially, the ZIP code of the Detroit River
