It looks just like one that I had longgggggg ago on my old 300. Don't quote me on this but I think it's an old Coralife brand (maybe Turbo Twist)??
If my memory serves me correct, it's the contact time that the water has with the UV light that determines what size aquarium it's good for, to an extent of course. For example, when I downsized to a 100 gallon I just increased the flow rate through the UV to compensate.
I don't think that you need to figure out the exact brand, just find a matching bulb for it. The Coralife UV bulbs look like that one in the pic. Good luck!