This thread seems fixated on "contact one".
I think thats missing the point.
When joining a busy airfield at say mid day for a fly in it is not unusual to have 10 or more aircraft in the circuit.
At that point you don't want ums and arrhs in the transmitions or "hello fred how are you today".
So it was common practice to say " I am in visual contact with 1 (or 2/3 ) ahead when joining.This told the ATC that you had seen the other aircraft and you will take your place in the que and not cut anyone up. The reply was "you are number 2 (or 3/4)" and confirm your place in the que.
The point was if the controller realised you hadn't seen ALL the aircraft in front he would automatically reply with " you are no. 6" .
The whole thing was quite condensed, "contact one (or 2,3,4,5,6,) ahead"became the shorthand version.
It was implicit in this message that you had seen the aircraft ahead and fit in without cutting anyone up.
I still use it, as a strip flyer that frequents fly ins that are often A/G only it lets people know you have seen them and they can consentrate on landing without having to wonder if the bloke behind is going up their chuff.
I think its a great shame its now a wonder to so many people and am somewhat supprised by this thread.
