Put an 8mm bolt in the end of the birf (as a handle) and push-pull the birf to see how much in and out play you have. If it moves much more than the width of the snap ring notch you have a long birf and a short flange. That allows excessive play.
R DOT appears on factory replacement parts that would have originally had the VIN attached to them. Original heads had VIN plates on them.
You will see this on other replacement parts that were originally marked. Such as fenders, hoods and doors.
I retro-fitted a factory 50 liter sub to my 80 ~20 years ago. It clears the factory spare tire crossmember. I have a very early first generation Slee rear bumper so my spare is on the back. The space below my sub is now occupied by an in-frame receiver trailer hitch.
The 80/100 factory subs are 50 liters and they occupy the open space above the factory spare.
Factory rear-mounted spares are independent from subtanks. I do not know the capacity of the 200 sub. I don't believe Prados have been offered with subs.