based on a quick search it looks like the 618-0319 618-0319 is the base hydrostat unit and likely includes only the Hydrogear internal workings, bare axle shafts and the cases. it is likely the 618-0319 311-0510 is the one specific to MTD family products yardman, bolens, CC, white etc etc and possibly others in the family. it will likely be the MTD product line specific complete assembly with wheel hubs, pulley/cooling fan and the linkages for the direction/speed and if it has a parking latch assembly it will likely be on the assembly as well.
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