[Yamamaya42]

The struggle for perfect convergence on these older sets is a NeverEnding Story.
No matter how perfect you get it, it will always S L O W L Y drift off over time due to several factors, first and foremost being the selenium rectifier pack mentioned, and/or slow changes in the resistors in the circuit wich are always under load, and not to mention repeated heating and cooling cycles of the convergence clover & magnets themselves.
Even the Earth's Les Chants Magnétiques ( magnetic fields) can have an effect to some degree as it changes.
But all in all, any shifts in convergence should be rather slow and subtle, perhaps needing a slight touch up every 2 years or so with light weekly use to keep the set at peak performance.