the older heaters did not have numbers and they worked flawlessley IME
.. and yes you had to adjust them by your thermometer.. its wise to do the same with a new one..
I think I understood that correctly..
when the old ones went bad they did not louse temperature usually they cooked the tank (they got stuck on).. soo.. its either your thermometer or the heater is undersized IMHO
I should note the older heaters that didnt have numbers that I used were Jebo's so that may have bearing on why they worked flawlessly.. :P