Hmm, interesting.
From the outer appearance, looks like it could be a Taiwanica cross.
The mix of trifoliate and monofoliate leaves also suggests it could be a second generation trifoliate hybrid, like perhaps US 852 x taiwanica, or Tai-tri x taiwanica.
Judging by the more tender appearing rind, I would think it less likely to be a cross from something like citrange or citrumelo.
Taiwanica appears pretty prone to resulting in hybrids, due to the plants vigor and many blooms, and the seeds apparently appear to be zygotic oftentimes.
(Eyeckr's "Ventura Lemandarin" originally came from a seedling from a fruit on his Taiwanica lemon plant)
It is also this could of course be a self-cross, where a Tai-tri fruit might have pollinated itself through sexual reproduction and there was gene recombination.
A cross with citrangequat may be another possibility.