MOTM

In Paul's MOTM 101 documentation of the vibrato circuit, he mentions that
it is based on an Electronotes vibrato circuit/article.  Which article is
he referring to?  Partly I'm curious, as there is an 'open' socket slot on
the front panel, I'm wondering if one could add a second vibrato output
that runs at a faster or slower speed relative to the existing vibrato
output?
thanks, Jeff