I have a 590 50663. The 51663 590-A1 is a slightly more robust version with heavier barrel walls, a metal safety and a metal trigger guard.
If you buy another 5xx series and later decide you want to purchase GRS and have them installed, you will discover that buying with the sights already with the gun is the only way to go.
In the Mossberg catalog when they describe a 590 50663 Special Purpose shotgun it states a "matte blued" finish and it will never be a bright blue, such as is found on typical blued shotguns and hunting rifles. It is a subdued dull blue finish. Think of a S&W Model 28 Highway Patrolman vs. a Model 27 revolver or Colt Python.
The Parkerized finish on the 590-A1 51663 is a matte grey/black with a very slight tinge of green to it.
Neither one wil shoot any differently than the other.