It was always blamed on thunder Ken but you're probably right. I think we all used the wet towel over the jug.
It all started when the cow had been milked. The warm milk was tipped into the tank at the top which had some form of filter incorporated. The flow of milk down over the ribs of the cooler was regulated by the cock on the tank. Cold water was circulated though the cooler and if done properly the milk was very close to the temperature of the water when it dropped into the Kit. If it was evening milk very often the kits would be placed in cold water overnight.
Around 1950 these became very popular. The Blow in churn cooler. These were popular because you could have the milk kit in the byre with a filter on top and pour the warm milk straight into the kit without having to walk to the dairy where the cooler was. When the kit was full you popped the cooler in, connected the cold water to it and turned it on. Because of the design of the nozzles in the hood which discharged the water after it had passed through the cooling loop the head rotated inside the hood and the water ran out through holes in the hood which dropped the water on the shoulder of the kit so the whole of the exterior had a film of cold water running down it.
The problem with these was that they depended on the temperature of the mains water and in summer when cold water was most needed it was at its warmest. There was no advance until refrigeration units could circulate super cooled brine through the coolers. (Or in the case of the Blow coolers, super cool the water that drove them.)
This initial cooling was the point where most could be done to delay souring and it was the weak point. Today with insulated and refrigerated bulk tanks the milk can be rapidly taken down to a very low temperature. That and enhanced Hygiene are why your milk keeps longer in the fridge.