Short answer: The 16c is modeled directly from an actual Scammell Dump Truck.
Detailed answer: A snow plow of this type is add on equipment for any truck. Dump trucks work well because ballast can be added into the bed for better traction. Since Lesney updated the Euclid quarry truck to a much larger model, it was not suitable for a plow of this type. The Bedford tipper could have been modified for a plow, but they decided to go with the addition of a Scammell dump truck with the accessory snow plow.
The Atlantic tractor had been replaced the year before with the Tippax refuse truck and the trailer was alone and waiting to be replaced. I'm not sure of the wisdom of pulling the Atlantic tractor before having something ready for the trailer's slot. Thinking back, the Atlantic tractor came along well ahead of the matching trailer so I guess they never were in sync.
There was not another dump truck in the line that could be fitted with a plow until the Dodge dumper came out. The Scammell lasted until '69 and was a good fit in the line but I guess another tractor with a dozer blade seemed more attractive to designers. Personally, I think the truck was a better seller and had more to offer a child (as well as the adult collector). From a collector point of view, I find the Case tractor with dozer blade (and plastic safety cab) pretty boring and with poor details (unlike the Scammell casting which was rich in detail).