That's a 590A1. Early 590A1 receivers were only marked 590. Rifle sight, parkerized heavy barrel, metal triggergaurd, metal safety and a bayonet lug = 590A1.
I think I might have you beat machman , Picked my first one up in '92' , receiver marked 500A , you can tell the really old one's from the square ejection port , new one's are rounded .
back then , IIRC , the only up grades you could get were Mossberg ghost ring sights and a adventures outpost sidesaddle which is the same as tacstar makes now.

Left one , 20" barrel ,factory Mossberg ghost ring sights , replaced front blade with Meprolight night sight , magpul barrel clamp [ the one for the remmington 870 fits the "non" 590a1 ] , Hogue overmolded forend , mesa leo stock adapter with endine buffer and magpul CTR stock [ I like my sling points as far back as I can , that's way not just a moe stock ] , the pistol grip is a cut down ergo AR grip......and some rattle can krylon .
Right one , 18.5" barrel ,started off as a pistol grip breecher , Mossberg ghost ring sights put on by Calgary shooting center , again meprolight front blade , vang comp +1 mag ext , a GG&G sling plate in behind that , just barely works with a magpul para clip , magpul stock and forend in fde of course , surefire G2 in a VLTOR light clamp and a mesa sidesaddle.