The 1946 "model" EC was sold into the 1947 calendar year. I think it was the first domestic "model" sold after WWII.
In the case of GMCs (not Chevrolet trucks), GMC simply did not use a year number, GMC used a model code.
For example: a 300 size (also called series) GMC was a 1.5 ton truck
EC and ECS and EF were model codes used in 1946 - here are EC-300 and ECS-300 and EF-300 model codes (and serial numbers):
http://www.oldgmctrucks.com/page0016.htmlIt that confusing enough?
It even gets better in 1954 - the serial number also has a letter-code preface (Z for Z-series).