Just bought a 78 from eBay. It's a single sided Zonophone serial X-46276 featuring Olly Oakly playing a tune called "El Capitan".
What I'm intrigued about is the description on the label. Rather than the standard "Banjo Solo" it's described as "Giant Banjo"!
I must admit it sounds pretty much like a regular zither banjo to me.
I've uploaded it onto my profile and attached it to this post if anyone would like to listen and tell me if they think they can hear anything unusual about it.