A Question about an Orange Tree .....

About ten years ago I bought a Navel Orange tree at the local Walmart garden center. I liked caring for it and the fruit has been plentiful and delicious. I added a Satsuma Orange tree to my yard and it has done well also. The Satsuma is a mandarin orange (Tangerine). I also have a Meyer Lemon tree.
A few years ago I decided to add a second Navel Orange tree to my collection so again I went to the Walmart garden center and selected a nice looking small tree in a pot. The pot had a label glued on which stated that it was a "Navel Orange". There was a plastic strip wrapped around the trunk which declared it to be a "Navel Orange" and the colorful dangling tag which hung from the tree said it was a "Navel Orange". That winter when I saw the first fruit it was immediately obvious that it was not a "Navel Orange". There was no Belly Button on the fruit and there were seeds. My first thought was to dig it up and throw it out, but I didn't get around to taking any action so it is still growing in my backyard. I have trimmed it back aggressively but it is doing well and looking healthy.
I have since discovered a very desirable characteristic about this tree and I am happy that I let it grow. The fruit ripens around Christmas time which is a few weeks later than my other orange trees but the fruit stays good and delicious on the tree for a very long time. It will be September in less than 2 weeks and the fruit is still great. This morning my wife went out into the yard and picked a few oranges and made juice from them for breakfast. She considers them to be more delicious than anything you can buy in a store.
The oranges are about 3" to 3 1/2" in diameter. Does anyone else have similar results with their oranges? If this is unusual, does anyone have an idea what type of orange tree I ended up with?
I think I will try to graft or T-bud a cutting from this tree as a form of insurance just in case something happens to this tree. I am in Shreveport,LA and every few years we have some winter weather that is cold enough and long-lasting enough to damage an orange tree.
Thanks for any feedback,
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