The Wells Fargo Building at 123 S. Broad St., between Sansom and Walnut Streets, has a distinguished history, from appearing on the National Register of Historic Places to starring in the 1983 comedy Trading Places.

But why does it have its own zip code?

Emily Boda, a mechanical engineer who has lived in Philadelphia for six years, wondered just that, and posed her question on Curious Philly, a forum where Inquirer readers can ask questions for reporters to chase down:

“It is the size of only one city block. ... Is there historical significance?”

Here’s what we’ve found out.

The 880,000-square-foot office tower, built in 1927 in the Beaux-Arts style with wide, arched entryways, is 29 stories high and was originally named for the Fidelity-Philadelphia Trust. In 1978, the building was added to the National Register, five years before it was turned into the headquarters of Duke & Duke for the Eddie Murphy-Dan Aykroyd film. Today it’s home to a Wells Fargo branch and history museum, as well as other corporate and nonprofit tenants.

Boda came upon the 19109 zip code while looking at the city’s OpenMaps website, an interactive map of the city.

The specialist maintaining that site’s data didn’t know the reason behind the one-building zip code.

(A Google Maps search for 19109 came back, oddly, one block north of 19109. OK, Google.)

So what’s the reason for the singular zip? We have a theory, developed from inquiries posed to a couple of U.S. Postal Service historians.

To begin with, 19109 isn’t the only zip code in Philadelphia associated with one property. According to the USPS zip code lookup tool, there are 37 zip codes in the city that are for a certain company, organization, or post office box.

“Unique zip codes are used for governmental agencies, universities, businesses, or buildings that receive such extremely high volumes of mail that they may need their own zip codes,” USPS regional spokesperson Ray V. Daiutolo Sr. said by email.

Even before zip codes appeared, this building between Sansom and Walnut Streets had a special status.

In 1943, the USPS was dealing with too much mail and too little staff because of World War II. So it introduced postal zones with delineated borders for 124 large cities, including Philadelphia, to sort mail more accurately. (The term postal zone previously had indicated how far a person was sending a letter.)

On Nov. 1, 1943, The Inquirer ran an article announcing the zones, including a map with the new numbers. Zone number 9: the Fidelity-Philadelphia Trust Building. The Terminal Commerce Building on 401 N. Broad Street also got its own zone, 8, as did the Land Title Building at 100 S. Broad: zone 10.

Daiutolo and USPS historians passed along documents that suggest why the building wound up with its own zip code.

In a postal bulletin from June 1943, the USPS wrote that postmasters should assign postal zones to each post office handling deliveries and mail distribution. And a directory of post offices from 1957 that lists postal branches and stations in Philadelphia shows that Fidelity and Commerce each had post offices that handled deliveries. The Land Trust Building still has a functioning post office.

The Fidelity station was on the sixth floor of the building, according to a postal bulletin from 1928. It’s unclear when it closed — it appears in a 1996 zip code directory, but not the next year’s.

So that’s why it had its own postal zone. But what about its zip code?

We can thank a Philadelphian for the introduction of zip codes. Postal inspector Robert Moon (1917-2001) proposed a three-digit code for mail in 1944 that would represent “sectional center facilities,” which process mail for large areas. He believed that this would make delivery even more efficient.

In 1963, the USPS finally used his idea for a five-digit “zone improvement [ZIP] code” system. Those codes launched on July 1 that year.

The first three digits became the code assigned to the area’s sectional center facility. For the last digits, "whenever possible these zone codes [from 1943] were incorporated as the final digits of the new five-digit codes,” USPS historian Jennifer Lynch wrote by email.

Philadelphia’s sectional center facility code is 191, so 19109 is that — plus the Wells Fargo Building’s postal zone. The Commerce building, now primarily a data center, got a similar zip code: 19108; the Land Trust Building is 19110.

Putting this all together, it seems most likely that the Wells Fargo Building has its own zip code because it once contained a post office that handled deliveries.

Correction: An earlier version of this post referred to a front-page Inquirer article from Nov. 1, 1943. In fact, that article ran on page 19.