I haven't seen this thread before, but I think your slippers were dyed black, there is a little overun onto the soles and one of the straps. Maybe they got stained and the owner wanted to save them, or they were later worn as evening shoes.
I've seen a few pairs of slippers and evening mules with this elasticated sling back and one of them was patented from 1917 for the strap, though they weren't that early themselves. The 1920's ones tend to have silk or ribbon back straps. I have seen later oriental embroidered versions. The pull tab, or peaked feature on the back of the strap on these says later to me, when this was appearing on day shoes.
I have a theory about the 3 digit sizing. I find vintage shoes often have more than one digit sizes. First the size number, then 0 for a whole size, 5 for a half size. It was suggested to me that the last digit(s) could stand for heel size, as dance shoes have. That wouldn't be the height but the surface width.
It can vary depending on where the shoes were made, and for sale in what country. What do these measure inside?
