That is called a telescope/telescoped comb. It looks just like you said, an innie bellybutton on the rear part of the comb.
It's considered a serious defect in showbirds. Other than that it doesn't affect the bird in any other way.
to clarify further, she has an telescoped pea comb. it is true that crosses between comb types often gives funky comb types. however, telescoped comb is a separate and specific genetic fault, not simply a result of mixing comb types. It can pop up in a purebred line and is very difficult to get rid of once it shows up.
Also it usually is visible on pea or rose combs. For some reason it seems to extremely rarely show on single comb, but there was an recent example on here. I suspect this gene can float around in single comb birds with no effect on comb but will show up if bred to a rose or pea combed bird..
