They look a bit similar to McKenzie Mitts (That image is probably of a reproduction pair, but you get the idea.) Here's a little blurb from a 1933 Popular Science magazine.

It seems like they were invented twice, once in 1925 by sheriff Jacob McKenzie in Kansas, and once in 1933 by a former Canadian Mountie. Apparently the design ended up being impractical - even though the prisoner couldn't grab a weapon, they had to be uncuffed to use the toilet, and there were rules about uncuffing prisoners during transit. Neat design, though.