Singer 222K Featherweights had only one optional attachment that was specific to the 222K and that was the Embroidery Darning Hoop.
The hoop came as two pieces.
The darning foot that is required to use the hoop came with the set of original attachments.
Although the hoop was optional the instructions for its use is in the 222K instruction manual.
The hoop came boxed and the hoop was wrapped with anti-corrosion paper.
There are two versions of the hoop. The early version was all chromed as is shown in the photograph above.
The later version was part black plastic and part chromed metal.
The hoop fits onto the 222K using the chromed small sliding piece that is in the "handle" of the hoop.
The attaching piece fits into the rectangular slot on the right side of the throat plate on the 222K.
The hoop has to be at right angles to the throat plate to fit into the rectangular slot.
When the hoop is turned to be under the darning foot the hoop is held in place.
Going back to the two types of hoops - the black version came out around 1959, about the same time as the red "S" 222K Featherweight, but any version of the hoop can be with almost any 222K. People with earlier 222K machines could have a black hoop if they bought the hoop years later.
Later 222K machines could have chromed versions of the hoop as many Singer stores would still have had old chromed hoop stock.
Both hoops work the same but in order for them to work you need a 222K Featherweight (doesn't everybody need a 222K Featherweight?). The hoop does not work on 221 Featherweights.