Hi!
I found some C/K Series trucks missing their model years, here's their links attached with my guess for their model-years.
1973-1980, i know there was 3 or 4 different updates between that years but this is according to their major similar appearance details (rounded body lines):
http://www.imcdb.org/vehicle_418938-Chevrolet-K-Series.html
http://www.imcdb.org/vehicle_52118-Chevrolet-K-Series.html
http://www.imcdb.org/vehicle_170351-Chevrolet-K-Series.html
http://www.imcdb.org/vehicle_544588-Chevrolet-K-Series.html
http://www.imcdb.org/vehicle_585381-Chevrolet-K-Series.html
http://www.imcdb.org/vehicle_585696-Chevrolet-K-Series.html
http://www.imcdb.org/vehicle_596030-Chevrolet-K-Series.html
http://www.imcdb.org/vehicle_646854-Chevrolet-K-Series.html
http://www.imcdb.org/vehicle_664552-Chevrolet-K-Series.html
http://www.imcdb.org/vehicle_290367-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_507906-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_554269-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_586148-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_586464-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_596904-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_636210-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_895343-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_376551-Chevrolet-C-Series.html
May be 1975 exactly for this one:
http://www.imcdb.org/vehicle_811595-Chevrolet-K-Series.html
Not sure for the Mexican models
http://www.imcdb.org/vehicle_329649-Chevrolet-C-Series.html
Not sure for this one too
http://www.imcdb.org/vehicle_459708-Chevrolet-C-Series.html
This could be a 1977-1979
http://www.imcdb.org/vehicle_632475-Chevrolet-C-Series.html
1975-1976?
http://www.imcdb.org/vehicle_750618-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_772987-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_776364-Chevrolet-C-Series.html

1981-1987:
http://www.imcdb.org/vehicle_767391-Chevrolet-K-Series.html
http://www.imcdb.org/vehicle_781727-Chevrolet-C-Series.html
i agree with rjluna2 assessment about the model-years:
http://www.imcdb.org/vehicle_673939-Chevrolet-K-Series.html
http://www.imcdb.org/vehicle_892040-Chevrolet-C-Series.html
Not sure for this one
http://www.imcdb.org/vehicle_641327-Chevrolet-C-Series.html


1983-1984 according to grill
http://www.imcdb.org/vehicle_28694-Chevrolet-K-Series.html
Another 83-84?
http://www.imcdb.org/vehicle_543021-Chevrolet-C-Series.html

Military CUCV M-1008? 1981+
http://www.imcdb.org/vehicle_374370-Chevrolet-C-Series.html

I'm feeling GMC here, if not! better to be listed as a GM C-Series by defaut
http://www.imcdb.org/vehicle_638657-Chevrolet-C-Series.html
Feeling another GMC at here
http://www.imcdb.org/vehicle_759702-Chevrolet-C-Series.html

I think these should be better listed as a 1973-1980 GM C-Series or C/K by default?
http://www.imcdb.org/vehicle_578422-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_255021-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_668716-Chevrolet-C-Series.html

Medium Duty as an extra-info for these models (1973-87 ?):
http://www.imcdb.org/vehicle_506374-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_529670-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_418244-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_582719-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_649334-Chevrolet-C-Series.html
http://www.imcdb.org/vehicle_659838-Chevrolet-C-Series.html