This is the rear bolt on the seat for a 3000 and others . I did not know what the plunger was for until I searched it here and found it is a stop pin for the yoke .
C5NN669C is $41 from my New Holland dealer , and I think 81837083 is just the pin ??? for $25 . Either way I would not want to buy a new one if I did not have too . I can probably reuse mine just fine but might make one from a bolt and keep the coarse threads on the top half , cut the pin on the bottom 1.870 inches by .300 diameter .
3.630 over all , 7/16 fine on top , 7/16 coarse on bottom .
Pic for reference . I had the lines lined up until the pin rolled , LOL