Kryptos has “SOS” in the Morse Code copperplating out front.

People gave it various meanings but it is essentially a distress signal.  “dit-dit-dit-dash-dash-dash-dit-dit-dit”.

Why would it be in Kryptos? 

Possibly as a reference to past uses of Morse code by the military and even domestic seafarers playing again on “public opinion”.  If this was his intent, I can’t really blame Sanborn.  Most of us think of SOS when we think of Morse code anyways.

So is it just that, a random historical reference?

Sure, maybe, I don’t know.  As far as helping with Kryptos though, I don’t think anyone has found it useful.

SOS, time is your position as the shadow forces the virtually invisible … of a lucid memory.  -R.Q.