The lock-down filler was the first of the vacumatic types.  A small blind cap covers a metal plunger, with a brass top. A small twist to the left will release the plunger, which springs up about 2 cm.  Immerse the nib in ink and press the plunger fully, wait a couple of seconds and release.  Repeat about 10 times (yes, 10!), pausing for a second or two at the top and bottom of each stroke.  Finish with the down stroke and twist the plunger to lock it.  Gently shake off surplus and clean the nib with a tissue.  Replace the blind cap.