Linda, My dh recently bought me a 15-91 in an art deco cabinet at our local lower end "antique" mall. We had seen it while walking through the mall and I thought it was a bit pricey. But then we looked in the drawers and it had all kinds of attachments/feet. Still, it was a surprise to me when it showed up on Valentine's Day. In the drawers were two darners. One looks just like the one Joe Miller is talking about .... oval with clamps to hold the sock. The other is the simple hoop one you mentioned. Sorry to say I have not tried either one of these yet as I am still working on cleaning up the machine. Anyway, the smaller one with the clamps describes as a "stocking and socks darner". The larger oval one says it is for "flat darning". So I am guessing that the second one is for mending holes in flat items like shirts, pants, etc.? Two different devices for two different types of darning would be my guess? Don't know that any of this helps you but there you go!