A twin screw is a tandem win which both axles are drive axles. Most tandems in the 50's and 60's had only one drive axle and the other was like a trailer axle. Eaton had a really crazy setup for a twin screw in the 50's in which the interaxle drive shaft went over the front axle. It was short lived and replaced with a through shaft similar to what Rockwell used. The Eaton setups are very rare. Not many were built and few of them survived It looks like this one may be a one or two year only three speed axle that used the old interaxle over the forward drive and shifted manually. By the early 60's Eaton used a thruoughshaft type power divider and the three speed Eaton became quite popular. By the 60's, the three speeds were shifted with air. My first Diesel truck had a 3 speed Eaton. The setup is comprised of two two speed Eaton Axles. In low range, both axles were in low range. In high range both axles are in high range Intermediate range is achieved by having one axle in high range and the other in low range. The power divider splits the ratio. The power divider uses a sun gear with planetary gears and power dividers were a very high maintenance item with 3 speed axles. I got my commercial drivers license in 1959 and still own a fleet of trucks. I have never actually seen a manual shift 3 speed Eaton. If it were in my neighborhood, I would buy it just because it is so rare. However, it will be very difficult finding parts for the Eaton. There is nothing different about the 2 speed and 3 speed Eaton except for the shifter setup. With a 3 speed Eaton behind a 5 speed a truck had 15 very evenly spaced gears. The drawback was the maintenance.
