I was listening to this interview with David Bossert about his upcoming books on the Disney Oswald cartoons.
http://www.disunplugged.com/2017/04/14/connecting-with-walt-podcast-041417/ It sounds like a couple more may have been found. David mentions that the number of extant titles is up to 20 (but he's only got hold of copies of 19 of them at the moment). By my count, that's two more shorts since the finds of Hungry Hobos, Poor Papa, Africa Before Dark, Empty Socks and Sleigh Bells. Unfortunately it's not yet been mentioned which other two cartoons have been uncovered and how likely or unlikey it is that Disney will be able repatriate both of these. I'm hoping this information will be revealed with the release of the book (which I've been looking forward to for a while now).
Unless I missed an anouncement, the "lost" titles were The Bankers Daughter, Harem Scarem, Rickety Gin, Neck 'n' Neck, The Ol' Swimmin' 'Ole, Sagebrush Sadie, Ride 'em Plowboy and Hot Dog. Hopefully we'll be able to cross one or two of these off the list soon.