A Dana 60 without a removable front cover? I'm not so sure about that. 35 spline Dana 60's crumble under Jeeps with tires in the 40"+ range. With Bobby Long's super axles and birfield joints they hold up under competition buggies, but his first breakage DID come from someone running those big Michelins like in the picture.
Look at the picture and notice how much narrower the Dana 60 is and how much smaller its tubes are than the axle under that bus.
On the otherhand, Rockwell and Marmon Herrington have been making front axles for big trucks forever. It's not a toploader Rockwell which is what people instantly think of when you say Rockwell, but I still think it could be a Rock. It doesn't have the distinct shape that the newer Marmon Herrington axles have. It could still be one of the older M-H axles with the more round centersection, but it's tough to say from that picture. My money is on it being a front loading 2.5 ton axle from something like an M211. They were rated for 12,000 lbs GAWR which would be good for a bus (my front axle is 12,000 in my bus). They are a reverse rotation design, but the t-case that came with them should take care of that problem.
If you have more information and can somehow explain to me how that is a Dana 60 or 70 I will believe you, but let's face it...either of those axles would be GROSSLY undersized in this application in every way...brakes....axle shafts....tube diameter and width....hubs....