I think Travlr is right it is either 12mm or 3/8" depending on how it fits, ie. either slotted or square.
https://www.amazon.com/Steelman-Retrieval-Mechanics-Cradle-Suspended-Vehicles/dp/B00FABVHWY?tag=cattletoday00-20
Btw, I am not sure about all of you city kids. We had no ice cream truck...