American organs are a kind of small harmonium, but as Smokey says they work differently internally. They were popular in the UK at the end of the 19th century and there are a lot of tunebooks for them. It seems to have been rather limited in its capabilities.

TheSnail: thanks, that book looks fascinating. The guy on the cover is playing what looks very like my metal Turkish G clarinet, only with less keys. It's a very good instrument for accompanying singing and is not at all raucous. It originated in France, as the "clarinette d'amour", and some of them must have made it to England.