Saw these on ebay, I know they are not the correct ones and not of same quality, but might be possible to utilise the red cover.
 
ebay  111496165208

I actually have red covers for the collet knobs, which look similar to these.  
 
What I don't have are the knobs from the original F2B preamps, which were more squat and had set screws.

Like these dusty babies?