Hi, just a little help please. I have 3 Gavita 750W lights that were bought at different times. It's time to change the bulb in the oldest one but i don't know which one it is........the only difference i can see in the lights is that one of them has a brass coloured knob for selecting the output and the other two have silver knobs. Would i be correct in assuming that this is the oldest of them? I'm trying to avoid buying a par meter at this time.......thanks.