Picture 2

Inside of the fruit.

Looks like some trifoliate hybrid rootstock like C-35 Citrange to me. Here is a link to that one but there are many other similar Trifoliate Hybrids used for rootstock.

http://idtools.org/id/citrus/citrusid/factsheet.php?name=C-35

Is the entire tree is this way and there is no graft junction near the bottom of the trunk?

If so, it is possible since you are in zone 8 that a Citrus tree froze down to the rootstock, killing the citrus tree and leaving the rootstock which has now grown into a tree. You would get the same thing if you cut one down into the rootstock and then let it grow back.

It will be completely inedible. I think I can pick out the characteristic triple leaf of trifoliates but I've included a picture that clearly shows the feature for you to compare to your tree.

I don't think there's a graft junction, but it might be that I just can't identify it. The triplet leaves are definitely there.

To the extent I understand what you're talking about, someone tried to plant a citrus tree by using a hardy but inedible kind of citrus as the root and grafting a different kind on top, but then whatever they were trying to grow died?

It's kind of a disappointment to hear that the fruits won't be edible, but I appreciate the help anyway!

I don't see a graft in you picture.

Almost all citrus are grafted. Many rootstocks are some trifoliate variety, many like C-32 and C-35 bred specifically for that use.

I don't know enough to tell, but it seems possible that the skinny trunk is your citrus and the main trunk is the rootstock. If someone let the rootstock take over, it could look like you picture after 10 years. Do the leaves on the skinny trunk look the same? The skiiny trunk is at the bottom in your picture.

I've attached a picture showing the graft union on a citrus with suckers forming on the trifoliate rootstock part. The graft union is the area on the trunk where it changes size at the middle of the picture. The rootstock can eventually take over. If something happened to the citrus tree, the tree could end up all rootstock.

Where are you located?

I think either of two scenarios:

1) The trifoliate rootstock took over after main plant froze or died.

2) This trifoliate hybrid was planted as a novelty and was not really meant to be eaten.

Are there other edible citrus growing in your region?

If you are in zone 8a, there are likely not too many good edible varieties besides some mandarins or Satsuma.

If zone 8B then this was more likely a rootstock for an edible citrus, but it died.