There were/are lots of liquid grain fumigants applied as you describe,some brand names were Weevil-Cide, Phostoxin and many others.  Application of these fumigants was more common many decades ago.  Some elevators would almost automatically apply fumigant to virtually every load to assure no infestation, others were more judicious in their use only applying when they knew they had a bug problem.  In part the extent of fumigant use depended on the aggresivnes of the fumigant salesman.