The arbor mounting hole on the old style deck has 4 spindle mount holes and the new style deck has six spindle mount holes.
To update the spindles/pulleys/hardware on a older unit to the modern setup (to fit a 80083696 deck weldment), you can see the following for help.
In general it's actually very interchangeable but we did update one spacer to make the belt angles a bit better vs what the old design had.
In short, it's the difference between arbor group 2990605 (old) VS 80087177 (NEW)
Broken down:
use 3 X spindles 1761761
use 18X mounting bolts 80006216
blades PNS are the same
use 3X 1656916 for blade washers and 3X 80020425 for blade bolts (torque to 50-55 ftlbs)
For the trim pulley side, stack up is: 80082639 low spacer, 1755470 lower pulley, 1758625 middle spacer, 1732951 upper trim pulley, 1656916 top washer, 80088554 trim topside bolt
For center and discharge pulleys, stack ups are same and are: 1737457 spacer, 1755470 pulley, 1656916 top washer,
(For all three top sides, you would match torque with blade side at 50-55ftlbs)
The arbor covers on the old deck will fit fine still, but there are new arbor cover versions in existence that you can see in 80087177 (no zurk hole on top).
Finally, to be clear, these extra new parts will allow deck weldment 80083696 (current kit 1687719YP) to work on older 52" tractor units with the old spindle setup.
Sorry that they arent service numbers, I'm sure Aaron is quicker than I at gathering that.