Will, your comments make me feel even more ancient, as I remember playing these instruments when they were new! Fenton-Weill did indeed enjoy healthy export sales, even in America. Sweden proved to be a particularly good market and over there some models were sold under different names, such as Zam-Master and Tux-Master, while the Twin-Master was nothing like the UK equivalent.
Talking of titles, I'm not familiar with the Duotone, or the similar sounding Dualtone, as these don't appear in any UK ads of the time or my company literature. Fenton-Weill initially favoured 'master' model names and the 'tone' types (e.g. Twistratone, Spectratone) didn't appear until around 1963, so presumably this means there must be paperwork out there I've yet to see. Such new additions to my archive would be more than welcome, as I'm well aware that although many of the pieces of the Fenton-Weill puzzle are now in place, tantalising gaps still remain, because Henry's handiwork was certainly pretty varied and equally widespread. In contrast, it's not too surprising that Dualmasters are comparatively common in the UK, as they were also sold here via mail order and accordingly became the final Fenton-Weills to remain in production, before Henry switched all his attention to the growing disco business.