don't mean to be argumentative or difficult, but...
the above linked listing is Mossbergs "Law enforcement" listing which are nothing more than the guns they either find work best for or SELL best TO law enforcement agencies. they are not the only one's with those features.
a few examples from the regular "500/590 Special purpose" page
#53660 590DAâ„¢ 12 gauge "Double Action Style" trigger, Pump Action, 9-shot capacity w/ parkerized finish, accessory lug, , 20" barrel with Modified choke, bead sight. ...MSRP $541
and two listings for the 590A1
#51663 590 A1â„¢ same as #50668 pictured above, except features heavy-wall barrel, metal trigger guard and safety button and field synthetic stock. MSRP $586
#51660 590 A1â„¢ same as #50668 pictured above except features heavy-wall barrel, metal trigger guard and safety button, brass front bead sight and field synthetic stock. ...MSRP $506
Note that NEITHER of these descriptions lists the lug anywhere, but the less popular DA version does, AND that these two A1s are the same exact stock numbers as the ones listed on the LE price list, that are noted as HAVING the lug.
also since that lug is now and i beleive from the begining, always been, part of that band it's on (ie it's machined/cast in, not welded/soldered on) i very much doubt that mossberg is going to take the time to make a third version of that part just to delete the lug. it's just too much trouble to take the thing off when the simplest way is to just leave it on ALL the 20" barrel, 9-shot 590s .