Shooting from the hip here, but this ("\xb0\x04\x00") could be interpreted as a protobuf with this IDL:

    message Foo {
      optional int32 bar = 70;
    }

that decodes to the following value:

    bar: 0

(or bar could be bool of value false, or enum at value 0/UNSPECIFIED, or int of any other size)

But with such a short message and without other samples it's difficult to prove or disprove that this is indeed a protobuf :).

One could try to set &pp=sAQB (bar = true/1), &pp=sAQP (bar = 15), ... and observe what happens.