The Parker Duofold Aerometric was introduced around 1950. It was available as a fountain pen and pencil, the pencil was often matched with a fountain pen as a set, but was popular enough to also be offered as a 'stand alone' item.
This pencil, a Duofold No. 3 in blue with gold trim, is in excellent working condition, it could well be new old stock.
The manufacturer's 'chalk marks', actually wax, were intended as an aid to the retailer and meant to be wiped off when the pencil was sold. They are very fragile and would erode rapidly as the pencil was handled.
The leads, which are accessed by removing the eraser, are fitted through the 'cone'. It has a smooth twist action, operated by turning the cap to expel and retract the lead.