The Standard Catalog of American Trucks has some info, but there's very little available for prod numbers on individual models before the 67-72 series anywhere - in 1965 427,100 trucks under 5000GVW were produced, 111,600 over 5000 and prolly around 1,000 would have been K20's judging by 67-72 numbers
if it makes ya feel better, it would be rare simply due to 3/4T's always being lower production, but a K10 long box would be rarer than K20, as short boxes were more popular for 1/2Ts - late 60's K20 production increased, K10 decreased
Bill
