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Turnstiles - also known as Quick Motion Brackets - and sometimes referred to as 'the bull horns on the head' - are part of the Spindle Feed driving gear train on the Heads of model 41B & 42B machines.
They contain a 'de-clutching' mechanism that is intended to de-couple the turnstile from the Slow Motion Hand Wheel (the round hand wheel) so that the Spindle may be hand cranked in and out of the Head without having to 'back drive' the other feed gearing.