I will be going to pick up an intact supposed running RD4 with a Letourneau cable operator blade next week and the seller sent me this picture of 3 other parts machines they have for sale. I am wondering if any of you can tell me what the big overhead sheave setup is used for? Never seen one and am not finding much of anything anywhere. 
Thanks 
CrawlerAddict 

First glance I'm thinking a cable laying plow. There's nothing in a 1940 LeTourneau dealers book that mentions anything that was produced like that.
Just the typical D4 Model E4 & WE4 Bulldozer.

  I believe a very short lived LeTourneau design. A picture from a group of LeTourneau file phots, provides a clear view of RD4 with the unusual design from 1937.

Hi, Folks.
From the which brian18a has posted, it looks to me to possibly be a variation of the varying diameter pulley that LeTourneau used on his cable operated scraper ejectors to maintain a constant spring pressure on the ejector return rope. It appears to have the lift rope from the blade going to a smaller diameter pulley on the side of the larger pulley.

Just my 0.02.

Thanks fellers. I was beginning to doubt myself when CrawlerAddict asked what I thought it was! Hey, Mr. C.A., just how many RD4's will you own now? Our editor should have gone directly to you for all the photos! Saturday meeting might really be a tractor intervention for some folks...hehehe.
I am proud to say that with Eric tapping into a new honey hole, he is going to have more iron than me very soon. Rumor was he bought a bigger trailer so he can "poach" them in fewer trips! Great find sir. Can't wait to see all of them in the future. JM

Thanks to everyone for the info and to brian18a for the great vintage photo. 
I won't mention any names but someone who has become known as the "Enabler" has been helping to feed my addiction. Unfortunately I will never have as much iron or the variety he does, but I  can still try! Looks like I'm going to be dragging these all back home over the next week. When I do I will post a bunch more pictures of the unique sheaved one.
JM it is true, I have invested in a much bigger trailer and better truck to hopefully become a safer addict 😆

CrawlerAddict 

I "resemble" that remark...Hah! Maybe I will get myself a dog too. I think Chrissy the pooch whispers in your ear while you are sleeping...BUY MORE TRACTORS...
Glad to see someone share the hobby with so much enthusiasm! JM

What this looks like to me is, another method of increasing the line pull for the blade lift rather than using the multi part block used on most cable blade lift assemblies.
It looks like the cable coming off the CCU attaches to the larger sheave, and the blade is connected to the smaller one, notice there is not the usual two part block assembly used to accomplish the speed reduction and increase power to blade lift.

Hi, Josh.
That was more or less the 'thrust' of my earlier reply but thanks for the further info. I noted the heavier cable from the top pulley to the blade which was what gave me the clue, similar to the heavier cable used to connect the apron to the apron lift sheaves on Cat cable scrapers.

Just my 0.02.