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.