The older one belongs to a Wahl-Eversharp Doric, a true classic American pen from the 1930s.
The plate acts like a zipper on the nib. Closed, on top, the tines cannot open. Open, on the bottom, the tines can give under pressure.
The small piece on top of the nib slides up and down along the slit. Placed on the bottom end, the nib is very rigid. On the upper end, the nib –free from the constraint— shows its maximum flexibility. This nib's system was patented by Wahl-Eversharp in 1932.
The Pilot Justus’s nib does exactly the same. This time, however, the pen owner does not need to stain his hands—the sliding plate is operated through a rotating ring inserted in the section.
The plate on the nib works simply by adding some resistance to the natual flexibility of the tines. this mechanism is less sophisticated than that of the Wahl-Eversharp.
The knurled ring acting on the plate, and the indication showing how to make the nib Harder or Softer.
The Pilot Justus on these pictures was manufactured in 1993 (December). This model reached some markets outside Japan.
My thanks to Mr. Álvaro Romillo (Wahl-Eversharp Doric) and to Mr Nozue (Pilot Justus).