It was a Borg-Warner transmission that had the OD built into the tail housing. The one pictured in the following article was for a Ford, but the one below that in the flea-Bay ad shows the Chevy version.
If it is like my 55 Ford OD I would not waste a dollar buying it. The only thing it did was free wheel going down hill.
All planetary gear sets do that. A planetary gear can drive only, it can not be driven (free wheels).
The reason an automatic offers any drag to reduce coasting is due to the mass of the rotating parts changing speed (angular momentum), and the front pump pressurizing the hydraulic fluid. Overall this amounts to 17% to 20% of the the engine's power being sucked up and wasted by an automatic.