You have to differentiate between "Triumph" as the name of a Sheaffer pen model introduced in 1942 and the typical conical nib of the 1942 Sheaffer Triumph pens to which is referred to as a "Triumph nib".
Jim Mamoulides at PenHero explains here that Sheaffer dropped the name "Triumph" from their model range of pens in 1949 while the term "Triumph nib" continued to exist for the conical nibs on their existing line of pens (eg Touchdown).
The conical nib was also found on some early Sheaffer cartridge pens (the 1957 Sheaffer "875" and "500" and the 1958 Sheaffer Skripsert and Lady Sheaffer). Period advertisements for the Lady Sheaffer praise the "exclusive wrap-around Sheaffer Triumph point", indicating that Sheaffer also used the term "Triumph" for the conical nibs on their late 1950s-early 1960s cartridge pens.
The price of the Sheaffer Skripsert with conical nib started from $2.95 in 1958, which was the typical price for a school pen of that time (and also the price of the Sheaffer Skripriter ballpoint) so I guess that the cheapest Skripserts came with steel (not palladium) nibs.
Sheaffer revived the name "Triumph" ca 1974 for a range of pens with inlaid or short arrow nib, that is the "Triumph 550" that you refer to.