Parker introduced this style of rotary activated pencil in 1958, the same year that a 'halo' was added to the cap.  This pencil has no halo so it would have been made early in that year.

Made in England, the pencil is in excellent working condition, and shows only very light signs of use.

The lead is extended and retracted by twisting the cap.  Spare leads, which are fitted through the cone, are stored in the barrel and accessed by removing the eraser.

The 51 pencil was sold as part of a set but also sold well as a 'stand alone' item.  It is still functioning perfectly after well over sixty years and should continue to do so for a few more decades!