What percent of FDA compounds have unknown mechanisms of action?

Typically several theories are suggested to possibly explain the
action of a drug. It would be rare that no one was willing to
speculate. Alternately there is typically a bit of doubt that the
mechanism has been identified correctly so perhaps 80% are in the gray
area with 10% each reasonably sure and rather unsure. Typically FDA is
more interested in identifing possible side effects than the
mechanism.  Neil