Living at the P.O.?

I was walking along Fourth Avenue yesterday, approaching 15th Street, when I saw a building detail I'd never noticed before.  In a row of three brick apartment houses, all of similar construction, the central building, 542, seemed to have had its cornice replaced by this:

Perhaps the Post Office inscription is a little hard to make out.  But what is it doing here?  I've found no mention of a post office ever existing at this address, only a couple of references to an accidental fall and a "trifling" fire in 1909,  and a resident a decade earlier delinquent in the payment of club fees.  Could it be just be an incongruous replacement for a lost original? Just a handy P.O. relic filling in? I suspect that could be it.  Oh for some research lackeys to put to work.  This one will have to wait for a while.