It has its own built-in 20W dummy load. You'll notice that there's 2-25 ohm, 10 watt power resistors in the back. They are tied parallel.
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Two 25-ohm resistors in parallel would be 12.5 ohms. If you put them in SERIES, you'd get 50 ohms, and if the resistors are the ones I think they are in the picture, they do appear to be in series, not parallel.