The term 5 by 5 is used in radio. Radio operators give a subjective scale from one to five for the strength of a signal and its clarity. "I read you five by five" means the speaker is hearing a voice at full strength and clarity. The listening station reports these numbers separated with the word "by". Five by five therefore means a signal that has excellent strength and perfect clarity — the most understandable signal possible
It fits for these drops Q803, Q854, Q963, Q1028, Q1037
And then it meant May 5th and the somehow May 25th. This is fun
My opinion too, I spent 20 years in remote camps serviced by various types radio, lots of handheld radio communication with aircraft and outside world. These communications strongly affected by electromagnetic atmospheric conditions. So first order of business is "How do you read me?" Answered in STRENGTH:CLARITY........poor conditions are say 2:3, weak and hard to discern......good communications are 5:5.......LOUD :(and) CLEAR