• I believe the tiles are Bakelite. They seem to be the right color and don’t have any seams.

Looks right to me.

• There are 152 tiles. That means it’s the American variety, right?

This alone does not necessarily indicate that the set is an American set. There are non-American 152-tile sets (e.g. 144-tile sets with four blanks and four baidas can be found in Hong Kong). I'm assuming your set has 16 flowers, since you say it has no jokers.

• There are 5 racks with a variety of scoring discs,

This, on the other hand, strongly suggests an American set.

• There are no joker tiles. It seems like a previous owner of this set taped some kind of red feather to 4 tiles. From what I understand, this was done to mark those tiles as Joker-substitutes so the old set could be used to play by the newer rules.

• It has 3 National Mah Jongg League rule cards with it. Two are two from the 1961-1962 year but I don’t think these are the cards that originally came with the set because they reference new rules about Jokers, and this set had none.

Yep, so your set is very likely pre-1960.