My 16500B has a connector on the front for an optional keyboard and it looks like the pigtail on that box would plug into it. -- If you look closely, the connector on the front is a "two dot" HPIL connector, which would plug into the "HIL" connector on the adapter, not onto the pigtail. All I can imagine is this plugs onto some later style HP keyboard with a 10 pin RJ connector (Frank and I saw a kb like this at a flea market a week ago) then you plug an HIL cable into that.