They dont, newer heads have a narrower gearbox than the older heads, but the switch was sometime after they started putting poly on. We learned this when we had a gearbox go out, got a salvage one from a poly head but it was the wider box and not the narrow box. You can put the wide box in and it will work but you kind of have to use some parts from the narrow one and some from the wide one, its not perfect but works in a pinch.

We do have a wide row unit if someone needs it.