(15 years on from the thread start - some fine old friends posting above who alas are no longer with us)
I've had a quick look around the 'usual suspects' and don't see a 1951 Avianca transatlantic schedule. Sometimes in those times other carriers would pad out their thin timetables by putting others, who they were maybe the overseas sales agent for, in as well, but Iberia, Air France, Viasa,etc, all have drawn a blank for this particular one.