N15-S5. Mitchell Witch Pen Set of 5.
Sizes: 1 (2mm), 2 (3mm), 2.5 (4mm), 3 (5mm), 4 (6mm) Square cornered, but with rolled over writing edge which allows pen to glide over rough surfaces. Works well on grained or coarse paper. Pen is a complete unit with nib, reservoir and plastic holder.