As some may remember, the '71 GMC 9500 I used to drive had a 637 Gas with a 10-Speed Road Ranger. To this day, I have never seen another. Here is one that is pretty close though. It's a '70 9500 with a 637 and a 5 X 3. Under the picture is a short story on the truck. There are also some pictures before this one and should be able to be seen by clicking on the arrows.
It looks like Madison County, NY bought 16 of these trucks all with 637's between the years of '67 and '72. Interesting, the '69 and older trucks has 5 X 2's (Trans and Aux) and the '70 and newer ones 5 X 3's.
Still no 10-Speed RR's though, but closer than anything else I've seen in the last 44 years. They are even the same color as my old truck, lol.
https://www.flickr.com/photos/919717...n/photostream/