The clutch friction plates are also called drive plates in the parts manual. They have splines that attach them to the clutch basket, which is driven by the primary chain.
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 Sportster clutch
does use an extra plate compared to the K-model, so the hubs
are different. The design uses 7 steels.
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 1965 parts book also calls out a wet clutch plate, 37985-57(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.