The label clearly indicates that the building was the ticket office for Pan American Airlines, if the presumption that "Pan Air" refers to the US carrier at the time. It would be especially fitting that the airline, which was such a prominent economic pillar in Liberia's economic surge which began after World War II, would establish its corporate presence in such a notable site, at the center of Liberia's business district, on Broad Street near the corner of Randall Street, which at the time was just opposite the Executive Grounds (prior the the construction of Capitol Hill, nearly all of Liberia's governmental buildings were aligned along Broad Street).
It's therefore appropriate that one of the most prominent economic connections to the United States, International Bank (Liberia) Limited, which is controlled by a private group headquartered on Connecticut Avenue in Washington, D.C., would today make use of the building as their headquarters and main banking hall.
Although smaller than some other retail banks, especially the large Ecobank office half a block away in the Episcopal Church Plaza (what was once the Chase Manhattan Bank building), IB Bank's wide reputation as Liberia's most reliable and trustworthy financial institution is reflected in its occupancy of such stately, historic building. Most likely it was a private home at some point beforehand, although its design suggests possibly that it was purpose-built for public use.
Although its deep porches have been enclosed, and recently the building was inadvisably painted in IB's butter-and-pea color scheme rather than the more elegant white, the mansion remains a gorgeous example of preserved architecture in the center of the capital. I'll have to try to research its pre-commercial history, take some better photos, and add it to the Architecture Tour.