I have a 1969-made 336 which does have that pin, as do the newer Remington 1895 rifles that I have (which are based on the 336). The lever plungers look the same to me, although I would not be surprised if they differ slightly due to different makers.
But, if your 1983 and earlier Marlin are both JM 336 rifles, then they should both have that pin. That wedged plunger needs something to lock under.