I have a 1953 99K manual, attachments are Foot Hemmer, Adjustable Hemmer, Multi-slotted binder, Edge stitcher, Gather Foot, and Ruffler. it shows the feed cover plate as optional.
I have a 1956 99-13 manual, attachments are Feed Cover plate, Zipper Foot (shows the fixed one with the slide for left/right), Seam Guide.