In my ham radio contacts from the early?'60s till rather recently, I often saw the slash?used to designate a zero; I noticed the same thing in engineering school in the late '60s and early '70s.? And, I have a Teletype Corporation teleprinter that has the slashed zero type pallet (ah, fond memories of loud noises and the smell of oil), although TTY Corp did make a non-slashed zero type pallet for the same type basket.? It was my understanding that, at least in this country, it was the zero that was slashed rather than the letter O.? Ah yes, more possibilities for confusion.