I found in a circular, distributed by the Treasury Dept., dated 1968, the information concerning the 53 gallon barrel. It was proposed to the Treasury Dept. that the 53 gallons should be the maximum capacity of a whiskey barrel. This petition was rejected, but their conclusion was that they would not set the maximum size for whiskey barrels. The selection of the size of the container suitable for maturing a product may continue to be left to the distiller.
So 53 gallons appears to be a quasi-standard. My only question: who proposed that to Treasury? Distillers? I do think Treasury was smart to turn down the job of setting a max size of barrels. Thanks, and thanks to the folks at ISC.