Yooper1

I am looking at a Gibson Country Western guitar that is said to be a 1956. It is round-shouldered with a crown in the headstock, so it has to be from 1956 to 1961. The serial number is 72773 11. According to my Gibson Flat top book, a 1956 SN should start with V. Apparently, any 56-61 should start with a letter. Any ideas about this one? Is this just a Gibson inconsistency issue, or some other explanation? Brent