Bruce Smith wrote

Eric,
Don't interpret the pre WWII percentage to equal an immediate post WWII
percentage. In preparartion for WWII there was a rapid conversion of
many cars in 1940 and 1940. In addition, the heavy tonnage and long
trains of the war resulted in a number of wrecks directly atributed to
too high a percentage of K brakes in the trains, thus there was a push
to convert even more. By 1944 the percentage was down to 50% or less,
iirc. That said, yes, some of these cars likely still had their K
brakes.
Regards
Bruce Smith
Auburn, AL