Not a trifoliata hybrid, otherwise you'd see at least some trifoliate leaves.

There's really no way to tell what it is from this photo.

A citrus expert in your region might be able to ID it from a sample of ripe fruit, taste of fruit, color of ripe fruit, skin texture and thickness, adherence of skin, number of seeds, as well as a closeup of the leaf and leaf petiole, leave fragrance when crushed, general ripening time.

It could be a grapefruit, or it could even be a rootstock, especially if it was not particularly sweet when ripe.

Maybe try the citrus forum? They really know their trees! Take some close ups of the leaves, bark, and any blooms. Also include your closest major city in the post to aid them.