The address in Melrose, MA has a 76 inserted into it. It's interesting to note that 7 days prior to the postcard's date the Post office implemented the 5-digit Zip code.
Here is an excerpt of the digits meaning.
By July 1963, a five-digit code had been assigned to every address throughout the country. The first digit designated a broad geographical area of the United States, ranging from zero for the Northeast to nine for the far West. This was followed by two digits that more closely pinpointed population concentrations and those sectional centers accessible to common transportation networks. The final two digits designated small post offices or postal zones in larger zoned cities.
And no, I don't work for the Post Office.
