Last year, I got some seeds from unknown fruit. Description of the fruit fits to citrus kotokan . Mandarin-like fruit, with size of pomelo. Seeds are bigger than pomelo seeds. Now I have healthy seedlings from those seeds.
Something interesting occures on those seedlings. On each plant there is one (on one plant - two), bifoliate leaves. On some seedlings, there are "double neck" on some leaves.

bifoliate leaf:

"double neck":

young bifoliate leaf

Great photos, I would bet that is some type of cross with trifoliata.

A very interesting specimen!

I bet for cross pollination with trifoliate too. Or may be with swinglea ? But I am not sure, that swinglea can pollinate citrus species. Never saw the mother plant, so I do not know, whether mother plant is hybrid, or the fruit comes from cross pollination.
Hope, one day will see fruits from these seedlings.

Also pumelo does not come true from seed.

Another interesting cross:
3 years ago I bought "Jaffa red pomelo" fruit (some said to be identical with Chandler pomelo) and sow 3 seeds from this fruit . I got 3 nice seedlings with different habitus. On the second one, I found big trifoliate leaf this year ! Hope, one day I will have the chance to test the fruits

Trifoliate leaf:

Guess you have some Poncirus or hybrid near you and the bees do the rest ...

Of course it has poncirus in ancestry, but not born in my garden. Seeds are from fruit, which I bought in supermarket - import Israel.

aha, misunderstood ...

nevertheless, it is interesting how hardly that will be ...