The exposed axle wheelsets were used up until circa-1978. They had overly large flanges. Also available up until then was the plastic covered axle model, also with oversize flanges. In 1978 the more common 'L' type wheel that we know today was released.
Regardless of values given in old 'collector' books, these early cars are actually worth less than later more common models. I sold off heaps of late 1960's/early 1970's models in mint as new condition and was disappointed with the lack of intrerest in these early, supposedly collector cars.