I agree with LONGBOX55.
As you might have concluded from the above speculation, the Chevrolet "VIN" buster is of no use busting a GMC serial number.
The Chevrolet truck body/cab ID-codes might have been called different names for different year, but in this case, for Chevrolet trucks, the cab/body code was called (by Chevrolet/GM) a
vehicle serial number in 1955-2nd and 1956. GM never called the vehicle serial-number/ID code nor the engine serial-number/ID code a "VIN" code; so, the name of these "VIN" busters adds to confusion by novices and seasoned owners.
As has been posted many times, in most states, until a state-VIN or a national-VIN was defined by law, the "VIN" used for titling/registration could have been the vehicle serial number or the engine serial (as long as it contained some sort of serial number). Some states saw to it that a serial number was stamped into the frame.
As far as GMC goes, the "chassis serial number" was the number on a plate attached to the cab (usually - at least from 1949 through ??) and the difference between a Z-code and Y-code GMC was a difference in model-styles: Z-code was the last of the New Design GMC trucks (1954 and early 1955 GMC trucks) and they were sold into 1955 (into March). The Blue Chip Y-code GMC trucks came out in March 1955 and were given the title/registration year as the year in which they were sold. X-code GMCs came out in 1956.
So, GMC made that 101PY Blue Chip Truck in 1955 (perhaps it was sold/titled/first-registered in 1956) and it most likely has characteristics that are different from those of a 1956 X-series truck. Check if there are any characteristics of your Y-series GMC truck
that are unique to 55 or to 56 Blue Chip GMC trucks). 
