Mystery Ship or Mystery S?

The Travel Air Type R was dubbed “the Mystery Ship” by newspaper reporters who weren’t allowed to see or gain any information on the airplane during the building of it.

Makes sense.

But what doesn’t make sense to me is why so many sources call it “the Mystery S”? Isn’t that sort of like calling a B-17, also nicknamed by the press, a “Flying F”?

If the airplane had been the Type S, it would make sense (“the Type S that’s a mystery…”), but I’m beginning to think that Mystery S is a misnomer that has over the years crept into aviation lore. Any opinions?