Just to throw a wrench into everything, the other instrument that I've seen referred to as a Melodion here in the US was not precisely a pump organ but a free reed organ. It had a keyboard on top and a spring-loaded bellows underneath that was hinged on the left edge. While you played, you'd let the bellows open, then press it shut again, which put the air through the reeds. So your right hand would go up and down a lot while playing the thing. I've only seen one of these once, in rural Pennsylvania. It had the word Melodion printed on it. I wonder if this the "missing link" between the pump-organ melodion and the button box melodeon...