Is Hohner Rocket Low key of F or key of C Lower?

Most likely the Rocket Low C is lower, i.e., C3, with the Low F being F3. You might want to try to blow up a print of Hohner's impossible-to-read tuning charts. F2, i.e., lower than C3, is usually described by makers as "LLF." Seydel makes low harps all he way down to LLE, i.e, E2. I have one of their Low A harps, i.e., A2. It's a real sandwich of a harp. (Hard to imagine playing a LLF or LLE unless I wanted to communicate with whales ... or submerged submarines.)