How do I tell which controller generation is in my preamp?

Early preamps with only LED indicator lights on the front panel used the original V1 controller. Starting with the V2 controller, all preamps got dual 2-digit 7-segment displays; the V25 controller, introduced around May 2017, uses the same display. A V2 controller briefly shows a 3-digit firmware number about 5 seconds after power-up; a V25 controller shows a 4-digit version. Starting in 2018, the menu-driven OLED display (V25 controller) shows version info as a menu item instead.