Probably a coincidence, but 0x555 == 0b010101010101. Just like the flip flop output signal.
I was curious and looked this up on Wikipedia [0]:
> It has been hypothesized that the 555 got its name from the three 5 kΩ resistors used within, but Hans Camenzind has stated that the number was arbitrary. [1]
[0] https://en.wikipedia.org/wiki/555_timer_IC#Design [1] http://www.semiconductormuseum.com/Transistors/LectureHall/C...