Wylie Williams wrote:

Beware oil from your hands. As I recall AR asked that their belt be
clean and dusted with talc.
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I wish I could remember for sure, but back in the recesses of the memory
is the statement by a representative of a then-independent Acoustic
Research, Inc., when the AR-XA was the current model of the turntable.
He visited our store and, among other things, said that the talc on the
belt was to allow a controlled slipping action of the motor when its
speed differed from that of the platter at startup, to let it get up to
synchronous speed without stalling as it tried to start the medium-mass
platter. It was a very low-torque motor, in a day when other-make
turntables, particularly "automatic turntables," had big, high-torque
motors, and high-traction idler wheels in a drive configuration which
wedged that idler wheel between the motor shaft and platter rim. If one
inverted the outer platter of an AR on the inner one, and watched it
start, one could definitely see the slippage between motor and belt at
startup. The motor would reach synchronism almost instantly but the
belt and platter would take several seconds to "catch up" smoothly.
Ever the experimenter, I tried observing startup after cleaning all the
talc off and reinstalling the belt, and did see a "chattering" occur
between motor pulley and belt at startup, so the motor was probably
going in and out of synchronism while this was happening. As the speeds
finally stabilized anyway, I couldn't say it made any difference in the
final reproduction.