Looks like a branch suckered off your rootstock. Eat one and if it is tasty keep the branch. If it is bad cut that limb off.

the sheer size hints of the vigor but that may be at the cost of flavor

Probably a rough lemon. Cut off the entire growth at the base.

Too big for a rough lemon - more likely a Cuban Shaddock. If there any trifoliate leaves could also be a citrumelo. Anyway, it's certainly the rootstock variety. Trace the branches back to where they join the trunk and remove them. It will be low down below the original graft line or even at ground level. You could leave the branch if you like it, but take care it doesn't take over completely. There's a danger it will grow stronger and faster than the orange which will be deprived of nutrients and slowly decline.

Thanks…. great info!

Andrew