When finding a new piece of Johnson gear, the knobs may be missing one or more of the white indicators. Items such as Q-Tip swabs have been used, but they do not look original and are made of compressed paper.
I found this information the web; it was posted by another Johnson enthusiast, AA8V, Greg Latta on his website detailing the restoration of a Viking Ranger I.
I use Elmer's rubber cement to locate these in the recess in the knob. It is normally used to attach photos in albums, repair book bindings, etc., so I trust it with my EFJ knobs.
Here is a comparison between the original Johnson indicator (Top) and the new indicator (bottom).
Here is a "Q-Tip" indicator. It looks out of place.
Here is the non-trimmed device used as the indicator.