Well the information on the Nova Resource page is fairly clear on the '72 SS. If K is not the 5th character in the VIN then it is NOT an SS.
http://www.novaresource.org/rpo_Z26.htm
I suspect since you didnt find the K in the 5th position of the Vin that it is not an SS.
EDIT: In '72 the SS Nova only came with the L48 350 motor. The engine code for the L48 350 is K. Since the only Nova that got an L48 350 is the SS if the engine code in the VIN (the 5th character) is a K then the car was originally an SS. If it is anything other then K the car was not an SS originally. The '72's are nice as it is fairly easy to determine if a car is an SS. On some of other years it is not that easy to tell.
here is the linkNovaResource.com said:
http://www.novaresource.org/rpo_Z26.htm
I suspect since you didnt find the K in the 5th position of the Vin that it is not an SS.
EDIT: In '72 the SS Nova only came with the L48 350 motor. The engine code for the L48 350 is K. Since the only Nova that got an L48 350 is the SS if the engine code in the VIN (the 5th character) is a K then the car was originally an SS. If it is anything other then K the car was not an SS originally. The '72's are nice as it is fairly easy to determine if a car is an SS. On some of other years it is not that easy to tell.