The clutch steel plates are also called driven plates in the parts manual. They have splines that attach them to the clutch hub, which is attached to the transmission input gear.
This clutch steel is the same as used on the 1952 Harley K-model. It is a dry clutch design that is enclosed and protected from the oil in the primary. The 1954, 1955 and 1956 K-Models and the Sportster clutch does use an extra two plates compared to the 1952 and 1953 K-model, so the hubs are different. The design uses 7 steels.
The 1965 parts book also calls out a wet clutch steel, 37992-57(7) that will fit the dry clutch bikes from 1954 onward.
These were bundled with the driven plates and a different backing plate in a wet clutch kit 37984-57
This clutch steel is used with the wet clutch design used on 1971 to 1984 Sportsters. The design uses 8 steels. The holes in the steels allow passage of a stud that connects the back of the plate deck to the pressure plate of the clutch. The frictional plates you use with this steel are narrower, to clear those studs, You can see the smaller frictional foot print on the used steel pictured here.
The 1971 to 1973 models used a nearly identical steel with a smaller hole where the hub studs pass through. This was abandoned in 1974 when three fixed spacers were offered by Harley.