This time of year I get photos of unknown citrus to ID. Most people ha planted a satsuma and the end up with something else. What happens is that the top above the graft gets broken off or freezes and the rootstock takes over.
The rootstock might be Swingle citrumelo (see below) or trifoliate orange (Poncirus trifoliata) – it looks like an orange but tastes terrible. There is also a dwarfing version called Flying Dragon.
I might have misidentified fruit looking like the one below because it highly resemble Ponderosa lemon.
So, if you get one of these ask what it was supposed to be. There is a very good chance it is the Swingle citumelo rootstock.”