Joe, there are several styles of back clamp ( I think I have four different sets). With significant differences, especially the tucker. Those in the picture are all the more later type, which is very similar to what Singer offered from then on in the side clamp versions. It seems that around the 1920's Singer "stabilized" their attachment design, and there aren't all that many changes after that.
Now that I have the W9 set, I can see how strongly that influenced the earlier back clamp attachments.