Generally old truck tires with a tube in them and a bit of tread left is all I ever see..... Works fine.
Likely they came with some sort of treaded tire on them, in that day something with bars, but not exactly the typical tractor lug - more angled & perpendicular bars/lugs not terribly high - was a common design. Many self-powered implements with small tires (swathers, etc.) had this type of design as well.
You can use cheap used truck tires, tractor tires ( run the lug patter 'backwards' to be accurate as the implement is pulled, the tires need to be oriented reverse to what the tractor tire pattern is) or search harder for a mixed or turf pattern that you like.
It will all work. Sorry I don't know size....
--->Paul