Actually they were rather common, though this one is in nice shape. Winchester actually chamberd the model 36 for this round in 1920. They stopped producing the chambering in 1927 and the shell some time after that. While the round has been around forever, the floberts never really seemed to catch on in the USA. When looking at old catalogs, it is much more likely to find 38-40 shot, 44-40 shot, etc chamberings than floberts. While the 22 rimfire shot was very common.
Until recently at least Fiocchi apparently was still producing new ammo for this. http://www.dixiegunworks.com/product_info.php?cPath=22_101&products_id=8250
This comparative pic is from the Wikipedia site: