Spool Pin. Normally a plain round pin mounted vertically on top of the Arm. Used to locate the reel of thread ready to feed in to the upper thread path. Some machines have just one spool pin, others have two, which allows one to be used to wind bobbins without having to unthread the machine. Some machines do not use a plain pin, On the early D9s, Wheeler & Wilson used an inverted 'U' shape, hinged at one end to allow the spool to pass under the other end. Willcox & Gibbs had a detachable 'mushroom' pin on all but the earliest of the chainstitch machines. The Free Sewing Machine Company used a revolving swivel sleeve around the bottom third of a spool pin so that if thread slips off the reel/spool of thread onto the spool pin the thread does not tighten and break or alter the tension.
Some spool pins tap into the arm of the machine, others screw in they can also be hinged making them foldable or retract into the top cover. During the 1960s Singer introduced plastic spool pins instead of metal ones on some models.