In 1981 MTD bought White along with a stack of other brands like Cub Cadet
Some time in the late 90's early 2000's they adopted the Cub quick connect deck lift across the entire product range and stopped making your deck
IT is NLA because MTD stopped making them , thank heavens
In fact I could not find an F (38") deck listed on any MTD mower after 1998 so I would guess they abandoned the F deck when they standardised on the Cub deck .
You might find some NOS ones under other MTD brandnames , Troybilt , Yard Machines, Bolens, etc etc etc
They will all have the letter F in the 8th position in the model number