Fairchild? (Large F ) transistors of 1970s

I was checking for data on an FT5154 transistor of 1976 kit. An old German transistor data manual showed it equivalent to 2N5154. A straight block of 50 or so others listed in that manual from FT1724 to FT5154 showed the same 2N equivalence. Anyone know of a general rule ? FT??? in hundreds don't seem to have direct number 2N equivalents

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