Isn't that a combiner and not a diplexer? From the tag it appears to be for those covert license plate frame dual antenna setups some alphabet agencies used to use. Can't imagine those working exactly great for any distance, but I guess that didn't really have to. A combiner is made to take all the power and split it between 2 ports, which appears to be exactly what that is doing. Not saying it's the greatest setup, but it is what it was built to be.