describes etaoin shrdlu
pretty well, only I've seen a linotype
machine. It's not a mark or a signal -- the letters e-t-a-o-i-n, s-h-r-d-l-u appear in that order on the keyboard. Linotype operators used to twiddle their fingers across these keys to see if the machine was working properly.
In that sense, etaoin shrdlu was the predecessor to hello world.