First off welcome to the forum!
While I'm not well versed in the flex, I'm pretty sure all the reciever/barrel parts are all the same as previous models... which means the only model that came with a bayonet lug would be the 590A1... which has a thicker barrel that is not interchangeable with other 500/590 models (except the breacher barreld 590's)
What I'm saying is... unless you have the thick walled barrel already installed on your 590... you wouldn't be able to interchange it for a 590A1 barrel that has the boyonet lug installed. I'm just going on memory here, so I'll edit my post if someone has more info than I do.