There seem to be two kinds of single discs for the old Elna machines. One is flat across the top. The other is shorter but has a little raised rim around the top side. These are actually a hair taller than the flat discs. The slightly taller discs do not go down all the way below the little spring loaded bearings which have to pop out all the way to keep the disc from sliding up the disc shaft. I wonder, if it is possible to sand off a bit of the disc to make them fit.