I have my D4 ready to hit the woods.  My 1956 D4 7U 36219 doesn't have any king of roll over protection.  I am sure there are correct names for these items, today it would be a ROPS.  Was anything offered "back in the day" so to speak.  How about a forestry cage with the hoops covered with heavy gauge wire?  Some kind of protection from dead fall limbs would be needed, heavier for roll over would be better.  Suggestions are welcome.  I am in Virginia.

Elton
 

I rebuilt my D4 canopy using the canopy in the parts book as a guide--i still had the 4 posts that i reused and built a new roof--but it is just a canopy and not a ROPS

FLECO cab
 

I have a rops for a D4D with brackets. It is a heavy unit. I can send pictures.

Here is a canopy with homemade sweeps on my D4D.  Wire mesh painted black helps with vision for the operator   

Don't put to much trust in any of the pictured canopies. A true ROPS would be attached to the frame not the fenders. The ROPS must support the whole weight of the tractor plus the impact as bounces down the mountain.
I have 3 canopies built like those pictured, before you start throwing things at me. I did a lot of weed abatement in old walnut orchards. They where to protect my head from limbs, worked very well. I had landowners say things like "I am so glad you have a ROPS, the last guy did not". I would just smile and nod. It was some steep ground by any standards, but crawlers had been working it for years by the time I came along.

The worst was the dirt had been moving down hill for years, so a lot more exposed rocks. Quartering up hill, top track on a rock as big as the D6, bottom track on soft shale. I way faster than saying it bottom track has spun a revulsion or 2 digging a deep whole. All of a sudden I have oil puck out the crankcase breather.

And speaking of sliding on a hillside, don't kid yourself there is no time to jump. I have been 5 to 10 feet down the hill before I knew I was sliding.

If you do build a full on ROPS, don't forget the seat belt. And then you need to have buckled at the time you need it. My neck is way to stiff anymore to watch what is happening behind me to be buckled down on the seat.

 

This is my 7U with overhead protection. The vertical sections and rounded over the top are 2" solid stock rolled at Struthers Wells fab shop in Titusville, PA in the 1960's.This canopy survived a trip off of a low bed at about 50 mph - only the sheet metal on the left rear corner was bent. When I added the canopy to this tractor, I made 4 45* angle brackets to the transmission case and the underside of the fender. The vertical posts go completely through the fender. 

A friend was unloading a 9U D6 with a shop made canopy when it slipped sideways and rolled onto the canopy. The fender on the left side rolled under and it sort of pinned him - did not kill him. 

I only consider this roof as overhead protection - limbs and things like that and rear protection from the winch operations.


 

Just to be clear it would be virtually impossible to put a ROPS on a machine of that age however a canopy to keep rain and wayward branches of is fairly easy.
The d4d is my favorite machine and crookedwrench has a really good looking tractor but when it comes down to it the 3/8 thick fenders will fold up like a bad card table if the machine goes upside down (no offense crookedwrench your machine is beautiful)