There are two gears left to discuss, reverse and overdrive. Overdrive is just like all the other gears except the countershaft gear is actually larger than the mainshaft gear. In an NV4500 the Fifth, or overdrive, gear resides in the tailhousing. When the synchro for the Fifth gear is engaged it actually locks the larger countershaft gear in place rather than the mainshaft gear, as the Fifth gear on the main shaft is a constant fixed gear. The massive cylinder on the end is a vibration damper, which many rebuilders dispose of.